What is wrong with my vehicle

I as well have a problem similar to every one else. My 94 suburban with 5.7litre 350 with TBI is acting very weird. It started about 3 weeks ago I was driving down the road when my motor started acting like it was running out of gas, but I had changed the entire fuel assembly 2weeks prior and had ran it on a trip about 200 miles round trip. I pulled into a parts store and started changing everything I thought it would be. I changed the Entire distributor assembly including cap and rotor , starter, coil, engine temp sensor, oxygen sensor , throttle position sensor, EGR solenoid, fuel pump relay, battery, wires to starter, in line fuel filter(again), alternator and ECM but to no avail no change. It runs fine cold but the warmer the vehicle gets the worse it runs with the effect that its running out of gas. Once it won't go anymore I pull over and turn off the motor and start it up again it runs like brand new until it gets hot again or I hit the break pedal then it feels Luke its running out of gas. I did change the speed sensor towards the back of the transmission but it got worse now it does it as soon as you start driving. I really need my vehicle and I have already dropped about $1200 into it with no resolve in site can anyone help? Is there another sensor I missed that I haven't changed? Also only a code 12 is showing up when I put a paper clip it the OBD the get the flash codes. HELP

You are the perfect example of why some people shouldn't work on their vehicles. Throwing parts at something you know very little about will only empty your wallet. Taking it to a qualified mechanic would likely have fixed it for less than the $1200 you have already spent. I would recommend you have a mechanic look at it. Best of luck with it.