This engine stalls often upon acceleration, in or out of gear. It is hard to start. I have to turn the key on and let the fuel pump build pressure and shut off several times before trying to start successfully. The throttle body has been cleaned. The engine had a valve job 10K miles ago and ran well until now. Timing belt and pulleys have been replaced and tensioned properly. Computer shows code 221 as fuel injection code and no ignition codes are shown. Could the throttle position switch be part of the problem? If so, how can I accurately adjust this switch? Otherwise, what are my potential problems and how do I diagnose them?