93 1500 drivability

I recently bought a used chevy 1500 4X with a 5.7. It runs great when started and drives well at first,but after about 7 miles of highway speed,it started to lose power and rattle badly. I shut down, and left it for about 45 minutes, restarted it,and it ran fine again, made it back to base with no recurrance of the problem. next day was the same story. I brought it to a shop that said the problem was my injectors,tuned up the engine and cleaned the injectors,and it ran great on the way home but the next day the same problem resurfaced. WTF? I feel it has to be something stupid and simple,but don't know what. Anybody can help?

Not enough info. A few thoughts however, was the fuel filter changed? How many miles on it?

If you have over 120k miles a good first step would be to change out the O2 sensor. They lose there effectiveness. You mentioned after 7 miles of driving. That sounds like when it warms up (engine goes to closed loop operation) it runs bad.

If you have high mileage and the sensor has never been changed, I would change it.

The truck does have 125,000 miles on the odometer. The fuel filter was changed...I will change the o2 sensor and see what happens. Thanks!

Ive got at least 270k on my truck and im almost positive the same fuel pump and o2 sensor that were on it when my uncle got it are still there. How important is it to replace these? haha I get like 4mpg and it runs like garbage but ive replaced almost everything possible with the ignition and the timing i fixed and so on. The only things left are the pump, o2 sensor and the coil that i suspect being not working or whatever. I almost am ready to give up on this old truck heh.

LT1, the O2 sensor is very important . The O2 sensor usually need to be replaced every 80kmi or so to be at their best. If its bad, you will get horrible gas mileage as your engine will probably be running too rich, as mine was. I changed it and got decent mileage again. Best part is, they are cheap. O2 will cost about $18 at autozone. Bad part is they can be a bastard to get off. Soak it in WD-40 overnight and that helps. ( nice honda comment! ) As for the coil. May as well toss one in for good measure if the o2 sensor does not help. It could be delivering spark but not really strong.

Also, if you have more than one o2 sensor, replace them all at the same time. If you have multiple sensors and one is replaced, then the other(s) will go out shortly after, causing the new one you just put in to go out.

And for ease of future removal put some anti-seize compund on the threads, makes life easier down the road when replacing them again.