So it sounds to me that your engine is leaning out. there are a few tricks that you can do to see if that is the case but i am sure that you already know them. One thing that I havent seen anyone mention is the needle valve. where the valve goes into the housing there is a O ring in there, sometimes when you rev it up the air presure will escape from this o oring. I know it sounds crazy but there is a really simple fix for this, take a small piece of fuel tubing, not the standard blue line (I use yellow) and remove the neelde valve. then reinstall with the fuel tube on the outside of the valve and the housing, its going to take some convincing to make go back toghter. Again I know it sounds stupid but my engine on one of my planes was doing the exact same thing. there was no fuel leaking out of the needle valve so I, like you was also stumped until an old friend showed me this trick........... it works. good luck bud