Help!

My son has 2 motors with the same identical problem, a Tower Hobby 75 (15 flights) and an OS 46 (60 flights). About 6 weeks ago the TH 75 started hesitating at throttle up, it recovers and flys fantastic until you have to do a quick throttle up again. He has learned how to DEAD STIK rather well lately. We've changed glow plugs several times. Now his OS 46 has started doing the same thing, exactly the same thing. Several of the guys at our small club have tried to help me tune the motors but with little success. The OS has 51 flights on it and has been "tweaked" only 3 times in the last year prior to this problem starting. I have changed fuel since it appeared to be the only common thread but still have the same results.

Any help would be appreciated.

David