Took the 12 up for another test flight this morning. I made no changes since last flight a week ago except to top off the tank with 11 gal of fresh fuel. The temp was 32C. The density alt was 2600'. Winds 5kts from 030 on runway 10. Static run up was 4950, but held that only for a few sec. and fell to 4920 where it held steady. WOT on t.o. and rpm at 5050 at angle producing 70 kts By the time I was at 400' alt the rpms had dropped to 4900. I lowered the nose to get rpms to 5000. Climbed to 2200 msl and leveled off for about 15 sec which produced only a continued 5000. I turned onto left crosswind since I decided to stay in pattern given what was happening. I pulled throttle to 4500 then went to wot and max rpm in level flight was 5300. I again pulled throttle back to 4900, got a burble which dropped rpm to 4700. I went back to 5000 with more throttle and then adjusted speed dropping to 3750 rpm which I maintained until turning final, and then adjusted to 2200 rpm for landing. No further events. I again checked my prop blades with my Warp Protractor (I know, I know, and I am building a Tony T. style digital setup).The blades were w/i .3 degrees. I am thinking now that the prop hub, adjusting, blade match, etc. hasn't anything to do with what's going on. Clearly it must be a fuel or ignition issue. I pulled the top cowl and inspected everything and found nothing obvious. No time to pull bottom today and check plugs, but will do tomorrow. Ok, please tell me what all y'all think is going on possibly. Thanks.