Toyota Repair: Temperature Gauge overheating, freon gas, temperature guage

Question
This is regarding my 1994 Toyota corolla DX 4 door. It has 140,000 miles on it. I bought this car in 2006 December

just for my weekend purpose. The car started anytime without problems. Not even a single problem for the past 1

year. I started to drive the car to work which is around 20 miles / 40 mins drive in traffic+highway. Few months

back, i had an oil change at a local gas station and freon gas fill up for my AC as it was less effective. Few weeks

back I noticed my temperature guage in dashboard going up and falling down. It doesnt raise rapidly but at a speed

of seconds hand on a clock. I dropped my car at pepboyz. They did the regular service and gave me back my keys by

saying they just topped some fluids, changed oil as its burning faster and changed airfilter. Now my car shows

hightemperature when i start to drive for 15mins and engine light comes on. There is no decrease in response of the

car or difference while driving. After sometime temperature falls off and engine check goes off. With your

expertise, Would you be able to suggest me what is going on?

Thanks
Raj

Answer
overheating problems are often made out to be harder than they actually are...when its overheating, you are either not making cool water or you are not circulating cool water.  there are still a lot of variables that i still don't know in order to diagnose it without touching your car.....are the fans coming on?  does it only do it in stop and go traffic?  if it overheats on the high way, you are probably not circulating water (possibly thermostat, plugged radiator).  if its doing in while sitting still or in traffic, check your fans....