2001 Pontiac Montana Pontiac Montana Problem

I have a 2001 pontiac montana, had usual gasket problems. Head gaskets just replaced along with other items (cost 2400.00). Since then car bucks & jerks on acceleration and sometimes at highway speeds. If you floor it on accelerating its o.K. I have since had the crank position sensors replaced. No good. Had a mech. Say to replace computer module ( 800 ) to start but that may not take care of it. Is it possible that timing was put off on re-assembly? Or any suggestions would be appreciated.