A/C And the engine coolant

I just got my 1999 pontiac gtp (used) 65k miles, and I m not sure what sup but like my car stays hot for like 2 hours after its parked in a nice cool spot, when its raining. Then I look at the engine coolant and it doesnt look like there is much but what is in there is kinda thick and brownish, is that normal? and another thing is sometimes when I turn on the ac while the car is idleing the my tank goes from F to 1/2 in like 10 seconds what could be the cause of this ?
thanks

The car uses Dexcool and should have an bright orange color. Shouldn't be thick or brownish. Have the cooling system flushed and filled. The other problem sounds electrical. Unless you really want to tackle that problem you may be better off having it checked my a mechanic.

I agree, Dex-Crap, oops, I mean Dex-cool turns brown and sludgy when it gets old or contaminated with green anti-freeze. Sounds to me like someone added green coolant to your car and mucked up the system. Do yourself a favor and flush the system of Dex-crap, (dang it, did it again) Dex-cool and replace it with green. I agree with 86TATpi that the gas guage problem doesn't sound like something you want to tackle yourself, let someone with wiring expertise get the headaches.

when i bought my car the coolant was brown and clumpy like mud, i had the person flush it before i bought it...

Agree. Totally flush your system (remove t-stat first) and refill with green stuff (all brands pretty much the same). Remember to empty and rinse the overflow bottle too. I've been running prestone for 4 months with no problems.