i have a question ? how old is the engine ?
does it need rebuilt ? if it does, that could be one of your problems. if it's the original engine with original miles, it may just be tired and may need a good rebuilt. if you need to rebuild it, then add all of the go fast goodies and you'll see a big difference. i got a rebuild kit from northern auto parts and block work done and it cost me all but 700.00 to rebuild. you may want to think about that along with all the other info you got from the other guys.