1999 Ford Explorer Never Ending Tune Ups

I have had to do no less than 6 tune ups on my truck in the last 3 years. I've actually lost count and it's probably higher than that. I'm pulling my hair out! I've taken it to several different places and begged them to figure out what the real problem is here, but no success. The last place I took it blamed it on a bad mechanic at the place before. But then it happened again. This time they took responsibility and replaced the plug and wires free of charge saying they must have been faulty. Now, less than 4 months later, my Check Engine light is on and the car idles rough with horrible excelleration. I know what they're going to say - tune up. What in the world is happening?

Here is my only un-educated thought: My truck never seems to warm up to "normal" temperature. The needle only moves to the middle of the gauge in the hot North Carolina summer. In the winter time it never pulls up below the cold line. Mechanics have commented that there seems to be a lot of build up on my engine. Is it possible that I really needed a new thermostat the whole time? Or is that the dumbest thing you've ever heard? My poor car, and my poor pocket book need your help! Thanks.