Electrical issue

[ninety4zj]

Hello,
I am new to the forum and hoping someone can point me in the right direction with an electrical problem with a 92 bonneville ssei.

Basically, none of the gauges in the instrument panel display correctly. What happens is the oil pressure is reading something like 120, while the temp gauge reads 0, sometimes maybe it'll go to 100 or so, and the fuel gauge either always reads empty or full. The strange thing is that if you were to drive on the highway for 5-10 minutes everything will read as it should, usually but if you use the brakes or turn on the lights it'll usually make the gauges read false. I would assume that either I have a broken wire to look for or a bad ground, but don't really know where to start. Any ideas?

Also, just curious, what kind of MPG should this car get, auto, 3.8l, supercharged.