Engine losses power or shuts off?

Okay, so, I bought an old truck from a “friend” loosely used... To fix up with my son when he turned 16, biggest mistake ever. He said the truck was good and just needed the engine rebuilt. I bought the truck and rebuilt the engine. The engine runs great. Test drive truck runs good. Once the engine heats up, the problem begins. Drive out of the yard right after cold start plenty of power and shifts fine. Once you drive and it heats up... If you stay under 2k RPM. Drives ok. More throttle, more crazy... “Around” 2k as downshifting to passing gear or up to 3rd shifting will not shift or shift correctly. If you let off gas and careful may shift or it may jump up and down in RPMS like it’s gonna shift but goes back and forth. If you step on it acts like you didn’t even press gas petal or like too much gas. Let off and it’s ok. Has died a few times during this exercise. Fires right back up no backfire or anything. When it’s acting up put in neutral and revs/runs fine. Only trouble is with load on engine. Tried running ac and other load devices to “strain” engine and no problem. I have changed trans fluid, filters, all three sensors in transmission. No change. Erg valve, checked for vacuum leaks, new fuel pump, compression check, coil and spark plugs. All good now. The one thing that I think it may be... Changed out hose from rear of engine to egr valve with “non-automotive” hose. Thinking it may be getting hot and collapsing? Causing vacuum problem? Gonna try replacing that if I can find replacement hose. Anyway took to a transmission shop. He said pressure sensor (which I already replaced) is throwing codes. He replaced, quick program... No change. Told me to bring it back when his boss gets back next week. More I think about it thinking maybe not even a trans issue. Help! Please!