1991 Chevy Truck Engine Starting to Get Weak?

My truck is a strong running motor for the most part. It has the 350 TBI engine in it. I have always changed oil religiously since I bought it with 93000 miles (It has 248500 on it now). It has awesome oil pressure yet, the same as when I first bought it. The motor is mostly still original, however recently with suspicions of the throttle body having wear issues on the throttle shaft, I rebuilt the entire throttle body. Replaced the fuel injectors, the base is completely rebuilt with new bearings and throttle shaft, and even the pod assembly is a brand new GM pod with remanufactured fuel injectors, and put a new throttle position sensor in. As far as I can tell, the distributor is good without any play on the camshaft.

However, I have issues with having a rougher idle when it is warmer or hot outside. When the ambient temperature is cold, it runs just fine. I am wondering if there is a possibility of something beginning to go haywire? Such as the cylinder heads starting to get too much wear? The fuel pump is fine, recently replaced the fuel filter and all the fuel lines (return and supply). Also installed a new cap and rotor, and spark plugs. The spark plug wires are good also. The engine temperature always runs normal in the middle. The engine never smokes (even at startup so the valve seals are good), knocks or anything. It uses about a half a quart of oil at 2000 miles. The engine is responsive and snappy, but just has a pesky rough idle at times that I can't solve. I don't have a service engine light on, or even an obvious misfire. I do also have to run 91 octane gas otherwise it "pings" and gets doggy.

Could the engine be starting to get weak on the top end or elsewhere? Any ideas?