Assuming the coputer controls the timing, you cannot get an accurate reading with the timing light unless you unplug/ground the appropriate wire.

Have you checked the codes in teh computer? any check engine light on?

Most likley culprit would be the TPS (throttle position sensor).

Randomly hanging parts IS NOT THE THING TO DO unless you have lots o money. The parts you told him to replace probably cost more than paying a shop to diagnose teh problem!