2001 Chevy Venture Cylinder 6 misfire

I brought the van in because of rough idle when at stop sign/light. Wife said the the car "jerked" at times while driving on freeway. My mechanic hooked up the car to get computer and received an error code, basically misfire, cylinder 6. He changed the spark plugs and the error code went away and it seemed fine. Several days later, it happened again. Mechanic got same error code, changed the coil (no charge), still got the code. Says it may be fuel injector on 6. I bought fuel injector cleaner to see if it helped, I thought it did, but the "check engine" light flashed several times and it was rough again. If not the fuel injector, what else should we be looking for. Pressure tests come back fine.