only 1 engine is doing it

what grounds have you checked ? dash gauges, wiring harness, computer connections. there is a large ground and power wire on the EFI harness check those #8 maybe, and gauges have a common ground buss that ties them all together, or it could be a wire shorting to ground causing this. a common way to test a gauge and wiring to the dash is to put the key in the on position and ground the wire at the sender and if all is good it will peg the meter. these can be a pita to find.

F-2 - The weird thing is that both gauges operate normally until the engine rpm reaches 3300rpm. Then the needles start going haywire all over the place. I've checked the grounds in the back of the engine where the computer and wiring harness ground is and the battery ground.

I wonder if its an over voltage issue when the revs come up ? alternator putting out to much voltage or a battery issue

Check your plug wires. I had issues with my oil pressure gauge jumping around over 3500rpm, turns out my coil wire was laying on the sending unit. Check wire harness crossover point to plug wires.

How old is the coil ? Possible connection and or wire or the coil itself.

You have MEFI 3 if all stock app and or if it has the stock ECM inwhich all 500 EFI s used MEFI 3 so no smartcraft.

check connections on the back of gauges

swap good gauges with your gauges that have issues and see what takes place.

Next possible corrosion issues causing the issue as I see your in Tampa.

Do you have the service manual and the correct updated material that goes with it ? Oem manuals really are needed for people whom work on their apps plus the needed wiring diagrams. if not watch out what you buy cause there is the outdated manuals that really are not correct to use.

Thanks guys and BUP - Good advice. Ill give check the places you mentioned and try swapping gauges and the coil. I don't have a service manual but Ill try to get one in the future.