1989 IROC Z 350 auto stock.

runs great till about 40-45 then hesitates like a stumble - will smooth out if I give it more gas.

Does this sound electrical or injector?

Where would you start.

Car has 42000 miles.

 

Runs great to 40-45 minutes after cold start? Or 40-45 miles per hour.

Have you scanned it for codes?

Have you checked the fuel pressure?

How old are plug, wires, etc.?

 

40-45 mph. original plugs and wires

 

Have you scanned it for codes?

Have you measured the fuel pressure - at idle, and with the engine under load?

O2 sensor ever been changed?

Injectors ever been cleaned?

Any chance the cat is plugged up?

Your driving at a steady 40-45 MPH and the engine stumbles? In what gear at what RPM? Is the stumble in response to a change in throttle position, or with no change at all.... just stumbles for no apparent reason? Does this happen at any other speed, or specific RPM?

When you "give it more gas", does it respond better to light or heavy thottle?

While your mileage is low, every component in the car is approaching 30 years old and suspect. Low mileage would seem to indicate it sits for long periods without being driven. Fuel can become contaminated and damage the pump, filter, lines, pressure regulator, injectors, etc.. How old is the gas in the tank right now?