My '99 silverado is having issues when started cold, or after being ran for a while, when it has been running at low rpms (under 1500) for more than 5-10 min. The engine starts quick and easy every time i turn the key and shows no check engine light, but throttle response is extremely poor. I can hear backfires and the engine has no power. It is most noticeable when a quick throttle response is required (i.e. pulling out into traffic and having to step on it a bit) Once the truck has been running at normal driving RPMs for about 5 or 10 min everything smooths out and runs perfectly fine. The issue has recently started showing up again if the truck has been running at low RPMs for a while, even with a warmed up motor, but completely disappears again once RPMs rise above 2000 ish.
Diagnostic attempts: I have changed plugs, wires, fuel filter, checked the fuel pump via a system pressure check, checked for an intake manifold gasket leak, checked my mass flow air sensor, even resorted to just running injector cleaner through the fuel, and am not sure where to go from there. Also ambient air temp. seems to affect it, I live in the desert and have had the issue at a minor capacity for a while, but now that temps are starting to cool down to under the 110s and 100s, it seems to be getting noticeably worse.
Thoughts: Only other very likely thing I can think of is o2 sensors, but would like to find an uncomplicated way of testing that without any fancy electronic gadgetry which I dont have. The pair of sensors run about $155 and I'd like to save that by figuring out if they're the issue before buying them. Next, the possibility of a bad injector, but once again, how to check the injectors in the simplest accurate way? The last and seemingly most unlikely problem I can think of is a coil pack is going bad. I'll be checking the coil packs by disconnecting plug wires and checking spark with a spare plug once my knuckle busting frustration drops a bit.
At this point I'm flat out frustrated with it so I'm taking a break and just trying to get some fresh thought from others.

 

If it was 02,s would get a service light. Sounds like a temp sensor .

 

IF you put some sort of cleaner through it, it could have
sent the carbon down and clogged your fuel injector. Your 02 sensors could
be bad, go rent a kit, remove them and inspect them before you buy them.
It could also be that your timing is off. The timing is off in my '94 Camaro v6 and its
causing hesitation, fuel injection problems, even EGR problems.. If you haven't cleaned your IAC valve yet it can get really dirty i would check that out, its usually on your throttle body with a sensor plug hooked up to it. Just clean it out with some carb cleaner and should help. Can't do any harm i suppose. I highly doubt its a gasket or leak unless your smoking out your exhaust..