Originally Posted by
KirbySER not sure if this is related, but if I am at a stop light with the A/C on, I will start to get a humming noise (I figured its the compressor), that goes away if I rev the engine a bit (turning off A/C also makes the noise go away 100%)
Check your fans. The one on the drivers side is the regular cooling fan, the one on the passenger side is the AC fan.
A bunch of folks helped me out (mechanical Newb) with a "checklist" of things to look at when I was overheating, and then helped me figure out I had a dead fan. The "checklist" might help you out.
http://www.sr20-forum.com/general-sr20/59748-fan-issue-mechanical-newb-questions.html
Maybe your AC fan is only working intermittently? That could explain the humming noise, going away when you turn off the AC, and it might explain the needle going up and down as the AC fan either works or doesn't work.
On the other hand, I could be way off base.