Fuel Gauge turned into Tach HELP!!!!

I have a 1986 IROC Z28. When im driving or take it out of park the fuel gauge starts moving like its the tach. After the tank gets below half a tank the gauge starts working correctly. Whats wrong with this. Also when i try to turn up the brightness on my dash it will studder(the light) and only get about dim. Any help with these problems would be great

its some sort of electrical glitch, either in the wiring or in the guage cluster, impossible to tell over the internet.

I have a 'used' cluster guage out of a 92' that works fine. It just has stupid white guages.If you need one. Dunno if 92 and 86's would be compatible.