It drives fine on tow/haul, but when cruising at a constant speed it idles and is hard on the transmission

I just purchased a used Silverado z71. i test drove and took the car to a mechanic and all seemed ok. now! it is idling at different rpms. sometimes too low and others way too high. around 1400 rpms in park. then when i shift into any gear it shakes the whole trucks and gets really rough. somebody please help me with the problem.

When I first crank up the truck it idles down like it suppose to, then it idles up high and sometimes it stays up high and sometimes it don't. There are times when I can drive it about 34 or 40 miles and it run smooth, then I can drive it about 2 miles and it starts idling high again and this makes the transmission shift rough.

Thanks for your help in this matter, Julius