Thank you for the additional information.
I am not terribly comfortable with a vague statement that they gave you of the compression being good. However, we will press on and look at some other things before considering that again now.
One additional question I have for you is were the camshafts/cylinder head removed?
My next suggestion would be to look for an air/vacuum leak on the intake. The easiest way to find this is start the engine (on the flush kit for cooling) and spray contact cleaner around all the joints between the throttle body and the cylinder head. If you hear any change in the idle, either revving up or wanting to stall, you have an intake leak. It is ok to remove the air filter/flame arrestor to perform this test. At idle, it will not affect the running of the engine. Do not expect it to rev up with the air box off however.
Something else that could tip you off about that is spark plug colors. Have you pulled them out and looked at them to see if they are all consistent in color? Which also brings to mind, have the spark plugs been changed lately? If you have any question as to whether the spark plugs are good or not, please take a picture of them in the order 1,2,3,4 Bow=1, stern=4 and attach to this thread.
If you have one that is a different color from the others, it can be indicative of a problem.