85 iroc z .if i turn on my lights it my dash lights work fine for 2 mins. ten my rpm and gas gauge lights go dim and also my volt gauge and oil gauge stop completely. if i dim the lights i start getting readings, but their not correct. so right now i drive it with the dash lights off. anybody have any ideas.

Bad ground? sort of woerking alternator?

the alt is still charging, i have a capacitor w/voltage gauge behind my seat.

Maybe your guage cluster is shorting out. Did you run any REM wires to your guage cluster fuse? Or maybe something is reading the wrong resistance? Oh and FYI, your capacitor might have a regulator in it which is cutting volts to be read on your LED screen, there for not giving you an accurate reading. Put a DVM on the battery with the car running, you should be getting between 13.5 to 14.5 put back into the battery with a properly working Alternator. Turn on your gauge lights during this same test and see if it fluctuates.