Loss of power

I recently rebuilt my Renix engine and I now have more power due to better compression and things like but as soon as it warms up I lose a lot of the power that I gained.
I'm thinking of going to a cooler T-stat but I'm not sure whats causing the power loss.
Any ideas?

dont go too colr on the t-stat because u will use more fuel.

did u change any sensors, or check them, sounds like a sensor problem.

Knock sensor could be reading "invisible" ping (internal engine noises like clicking lifters which sound like preignition) and retarding timing, cutting back power. How's the cat? Did you bore it? Shave the head? (increased compression brings ping and computer controlled timing retardation along with power drop)
Rings not broken in yet? (up to 10,000 miles with some brands)
There may be other issues too. (like bad O2 sensor leaning fuel out too much)

Well I'm not sure about the cat, I'll be doing the exhaust soon so I'll check that but I dont see how that would effect it seeing as it only happens when its warmed up.
I didnt shave the head, or bore anything out. I just honed the piston walls to remove any ridges,new rings,f/r seal, and every gasket possible with everything tourqed to spec. I'll check the O2 sensor though, thanks.

The compression is dropping unless there was no wear, ridge or cylinder wall taper. That's a re-ring and not a true rebuild.

Well yeah, your right, I just got used to telling people that I was rebuilding the engine when they asked why it was off the road, it's a lot easier than saying "I'm doing seals,rings,walls,gaskets, and different but stock intake"
And there is still a taper, just got rid of anything inside the walls.