1996 Chevy Tahoe Sluggish after valve job

Hi,

I just had a tune up, valve job work and the fuel injectors replaced on my truck, but when I got the truck back. It was sluggish on hills and slow to accelerate. I kept on telling the mechanic who did the work and he kept on telling me that the truck is fine. So I took his word for it and drove it like that for 3 mos. Then one day the check engine light came on with a misfire on cylinder 8 and a P1351 code. I took it back to the shop and he did a compression check and test drove the truck. The mechanic couldn't figure out the problem. I finally gave up on him and took it to another shop. The new shop discovered that the timing was off by 12 degrees. They fixed the problem and truck ran much better. But now it occasionally would run sluggish. In the mornings it will run great but once it starts to warm up it will run sluggish and I really have to push on the pedal to get it going. There has been a faint burning smell and when I open the door after I have driven it for a while. I can feel heat coming from the bottom of the driver side door. I also noticed that the temp would go over 210 then slowly go back down to about 160-180. What could be the problem? I need to make this vehicle last for another year or more so any help would be greatly appreciated. Thank you