96 suburban power loss at high rpm

I have a 96 suburban with 172,000 miles on it. I have been noticing a power loss when I floor the accelerator and it gets to a higher rpm but if i let my foot off the pedal a bit it shifts into second and goes. . . Ok so that was the problem up to yesterday, I drove the truck for about an hour and a half and I noticed while I was on the freeway and I tried to accelerate I actually lost speed, while this was going on my check engine light came on. I pulled off the freeway and I had a problem even accelerating to 30 mph. I got it home, let it cool down a little bit, and took it for a drive. The torque was back but it still had the previous problem when I floored it. I brought it home, parked it, and let it sit over night. I got in it this morning and had no problems at all, it accelerated and shifted perfect so I have a feeling it is an issue when the engine gets hot. I just replaced the plugs and air filter last month and it helped the problem for a bit, I am going to replace the wires and the plugs todayand hope that it was just a set of old wires causing all the drama. The truck has no cap or rotor so I dont know what else it could be, any help would be greatly appreciated. . . Once agian it is a 96 chevy k1500 suburban. Thank You.

Check your basic tune up.
How is fuel filter and fuel pressure?
Any engine miss or check engine lights on or flashing?