Engine stumbles randomly at highway speeds and at Idle. A code 42 is stored. When the codes are clear the engine runs smooth. But will randomly every 5 to 10 min stumble. if at idle it may actually die and has to be restarted. I don't have an oscilloscope to check square waves as I read in a troubleshooting guide. The previous owner may have been chasing the same problem. I have receipts from the dealer Capital Chevrolet in Austin Tx. they took it to and replaced both fuel tank pumps, selector valve, tank selector switch both TBI injectors,distributor,ignition coil, TStat, Alternator, IAC valve. Will start and run just fine but will randomly stumble has me puzzled.