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 no 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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While it may be the thermostat, it is also possible that the radiator is getting sume buildup or some tubes are clogged and not working as well. It is also possible the sending unit is bad. Mine used to peg itself all the way up even though it was at proper temperature.

My LeSabre transmission did that. One day is just stopped transmitting all together and I was able to limp to a shop. Get it checked by a pro before it gets worse. $2000+ later is it just fine. Hope you fare better.
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