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.