Engine knocks

My engine started developing a tap on the top end. Tap got louder and louder. I knew it was the camshaft hitting a dry rocker arm. I had the engine pulled and it was confirmed one camshaft lobe was slamming a dry rocker arm.
I had the top-end rebuilt. New camshafts new rocker arms new lifters new timing chain kit new water pump new rear main seal all new gaskets.
Mechanic called and said it was repaired. I came and picked it up but after just a couple miles the engine shut off on me. Started it back up drove it (engine knocking hardly any power) a couple miles back to the shop to have them look at it. After two days they called and said I needed to just put in another used engine. Wow!
So, when I start it up it sounds rough and noisy but within 1 second it sounds great runs quiet idles nice. In fact I can let it idle for hours. When it is still cold I can drive down the road 1/2 mile to store and back no problem.
When it gets to operating temperature the engine loses all power and sounds like it's going to grenade (to me it sounds like the timing advances so much it runs like that). After a couple minutes I can start it up and it idles just fine but as soon as you hit the gas.
The oil light and engine light are on.

the second mechanic I have taken it to is telling me the engine just needs to be swapped out.
I don't get it. My engine runs great when it's cold but because it runs rough at operating temperature it can't be fixed.