Looks like you've done a pretty good job running down many of the likely causes.
It also sounds like you've put in some time into troubleshooting the issues with some logical testing
Your test of grounding the fuel sender wire was
exactly the correct way to "quick test" the whole system
The gauges themselves rarely give problems (in fact i've only ever seen tach problems in these clusters)
They are of an analog design, utilizing wheatstone bridge circuitry to "measure" things by varying one leg of the bridge and comparing it to the known legs (this leg is connected to the wire running out to the sensor)
full technical explanation / theory
...before glass cockpits in new aircraft, this is how all the gauges in aviation "worked",and they were
very accurate
You don't really need to know any of this, it's just background behind the reason your earlier "ground the fuel sender test" was such a valid way to show that the gauge is receiving power, and the wiring is intact enough that it';s time to investigate other possibilities
It's also worth noting that the engine temperature sender for the gauge is a single wire unit, totally seperate and not connected in any way to the two pin engine temp sender the ecu uses
Does any of the wiring behind the panel look like it's been tampered with?
Sometimes, some scabby work can be done by shops/owners installing aftermarket gauges, stereos and the like
Usually, most of that carnage is down behind the radio, but i've seen some dedicated mooks bodger things up behind the cluster to add mad "a" pillar gauges as well. You're looking for signs like extra wires not in the loom and the the loom not secured / being out of position. In addition, lot of these cars have had the dashes out, and the properharness routing may not always have occured upon reassembly. These kinds of problems can take months/years to show up, but eventually out of position harnesses will strain individual wires or cause chafing against sharp dash bracketry, and
bada boom weirdness manifests.
The vr4 cluster keying into fixed plugs can (and does) create some problems with alignment when it's removed and re-installed. It's a robust design, and rarely gives problems in service, ... but impatient bogans can cause many problems
when they are wrestling with the speedocable, and things can get bent / damaged / spindled and mutilated. A few moments
carefully inspecting the contacts of the harness and the circuit board contacts in the sockets of the cluster may find an anomoly. I know you said you looked at it, but take your time and
carefully inspect the board traces where they contact the harness pins, they can get rub thru's that are hard to see ( a 10x magnifying glass is very helpfull here)
There is one other avenue to investigate, but it's uncharted territory for me on these cars
I only mention this because of the commanality of BOTH of your gauges showing errant readings
following info NOT verified on the vr4 platform!
The systems I'm familiar with (in aviation) use a voltage stablizerin each gauge unit to cut gauge system supply voltage down to a level well under nominal system operating voltage. This is done so sagging voltage due to an alternator/generator failure will not cause errant gauge readings untill the voltage gets waaay wonky. Aviation engineers design
very robust and accurate systems that will work under adverse conditions ... it would totally bring the suck if you were showing half tank after shutting down an engine ,only to flame the other engine out over the tasmanian sea due to fuel starvation
... automotive engineers ... well, they care more about saving a nickle a unit ...
Many german cars i serviced back in the day used a seperate, stand alone voltage stabilizer on the back of the circuit board. (little three prong jobber) This was used to manage gauge supply voltage to all the gauges on the board from one common unit to save the cost of individual stablizers in every instrument casing . It was common failure point in the late 70's and 80's and some manufacturers had issue all the way to the 90's
... many was the time replacing the
three prong magic pixie wrangler made everything golden.
i just peeked at a vr4 cluster, and i do not see a standalone unit on the back of the board, so it *may* not be used in this application. If i get a chance later, I'll see if i can find my service manual for this platform and look up the diagram to see if it's a potential player
Sorry i couldn't be more help, but hopefully, one of the smarter guys will be along in a minute to help you resolve your issue
Cheers!
[edit]
Can you borrow a cluster from a friend to perform a test of substitution?
it's a quick and accurate way to narrow things down
How's your charging system / battery health
Are you noticing any other anomolies when you are having this problem?
If the battery voltage is fluctuating, weird things like this can happen
And, lastly, grounds, grounds, grounds
Anytime you are having weird electrical faults, a good starting point is to clean each and every ground everywhere in the system. ... and they are in random places on these cars.
I'm not sure where the tech library is now, but Mr Fixit did an awesome job catalogging electrical diagrams, including ground locations.