1994 Buick Century

i just purchased a 94 Buick Century about 2 days ago and now starting to see some problems like temp. gage is pretty close to the red supposed to be at about half should i check thermostat or could it be
an imternal problem?
The other thing i noticed is sometimes on the highway overdrive is not working, tranny working great no slipping at all but sometime on the highway for no reason it shifts back into 3rd gear i know because while driving i play with shifter between 3 and D , there is no difference in rpm oh did mention that the check engine light is not on, well its not anyway I think my problem might be that i need to change kickdown cable. If any 1 has some solutions to these 2 problems other than what i mentioned, or let me know if im right about the thermostat and the kickdown cable.
THANKING IN ADVANCE
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A backflush, where you hook the garden hose up - with one of those heater hose kits, of the cooling system every 2 or 3 years is a good idea, that's a good cheap start to look at. On my 96 Buick Roadmaster Station Wagon the temperature gauge goes up rapidly when sitting still or in rush hour traffic, below 30 mph. Both electric fans turn on when the gauge hits the red zone and the car cools off, unless it's 95F+ outside. Turn on the air conditioning and both fans will turn on.
I have had 2 GM cars where is dipstick is not accurate. I had an 89 Caprice and for years the transmission was above the full mark, I just left it alone. Then it leaked down to the full mark. On the 89 chevy when the transmission stick was at "full" overdrive would not work ... at the $1200 rebuild the transmission shop said the transmission was burned up. When I got the car back it was above the full mark. On a 97 Buick Lesabre, the transmission overdrive does not work when on the full mark. Pick a point on the dipstick about an inch above the "full" and let that be your (full) mark.

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