thielsr5

Ok, I've done my fair of research and at a complete loss.
Here's the issue:
-When in first gear at high RPM's, then shift to 2nd and bucking starts, only when engaged initially to 2nd gear...
-Also it bucks a little in 4th/5th gears at no acceleration at cruising speed.
-when I don't drive the truck for 2 days or more, then after letting it sit for a few days, it drive so perfect! no jerking or bucking. But driving it consistently for a few days the bucking starts.

I've taken it to the mechanic and of course it runs perfect for him! I'm waiting for the problem to get worse so I can have the mechanic actually fix it.

Could it be a valve adjustment? Cause the guy I bought this truck off of said the engine was rebuilt and needed a valve adjustment.
Is it that simple?