'89 w/EFI dies at idle

Sorry I don't have the engine specs --(my daughter's truck) She just move to Chicago and reported that the "check engine" lite started blinking intermittently with the only noticeable performance lose was when they would come to a stop --the engine would stall -- started backup and everything seem fine(lite still blinking). About 100miles later,(when just coming into Chicago) severe loss of power, and the and engine is very hard to start. They dropped it off at a service garage where they were told the intake manifold gasket was leaking!!!??????? Well--- $320 later with new gasket the engine did the samething leaving the shop and 4 hours later, the truck still wasn't right. (personally I don't think these guys had a clue!!)
The garage could not find any error codes in the computer.

Any suggestions on what sensor has failed -- is there a "Mass Flow Sensor" flow senor on the intake system that needs to be checked.

How about a garage that they can trust???

Thanks

The engine turns out to be a 5.7 lilter