Key word is he "thinks". They did not fix it so he does not know... I would not put much stock in him at this point.
What sends the signal to the computer to fire the coil and or pulse the injectors? I think you should have a sensor mounted at the transmission mounting flange. If the signal is lost i would expect a code. The computer recieves reference signals and merely interpets the info. No codes stored either? Check pins and grounds wiggle connectors, check grounds as previously stated.
Recently had a GMC that was not injecting intermitantly. A bad temp sensor was the cause. Drive it with a scanner hooked up and monitor refernce signals