Before I pack her up and send it our for service, I am hoping someone with experience can suggest something that I have not already done. My 250, which has always run very strong, over the last several flights has lost rpm when at higher and full throttle. Now it is at the point where it will shut down completely after passing half throttle. As the engine heats up, the rpm lessens even at lower throttle settings. This is what I have done to troubleshoot:
1) confirm fuel lines in tank and lines from tank to carb are good.
2) confirm all bolts are tight securing the muffler ring, as well as every single other bolt anywhere on the engine.
3) opened carb and thoroughly cleaned it out. Had some gunk on the screen filter.
4) removed carb from back on engine and cleaned all gaskets. Then put on some gasket sealer and re-installed the carb.
5) confirmed all valve covers are properly spaced.
6) changed ignition module to brand new one. Problem did not go away, so re-installed original ignition module.
7) confirmed ignition battery is fully charged.

I'm stumped! Any ideas? Thanks.

Are you using an external fuel pump as well or the engines own?
I've heard of issues with the line getting blockage if not cleaned regularly.
I have a 250 as well but very low time and I haven't had any issues yet.

Was running the external fuel pump when the problem started. I bypassed the fuel pump to see if that was the problem. It isn't.

What about your fuel oil mixture? Sort of sounds like its seizing up. Does it turn over easily by hand cold? When it quits hot can you turn it over easily?
Also, are the plug colors nice and tan or black/wet?

Sounds like you have some rings stuck in the piston grooves. Gum buildup caused by oil and varnish. It will cause compression loss when engine gets hot.

Or you could have a couple of valves that are stuck also.

Thanks for the input, guys. I packed it up and sent it out for service this morning. I'm pretty handy, but I also know my limits. We shall see.

You said you tightened all the bolts on the engine - the chrome hex cups that hold the valve pushrods can be over tightened and cause the lifters to bind.