I had this setup to start with...advice from the feild experts...
"glow engines produce alot of oil and residue that at low idle will pool up and keep the glow plug from heating up....never put a two-stroke up-side down." (or something like that).
So I went on home and placed the engine on its side. Ran like a champ from then on.
To do this I had to take a dremmel and do alot of cutting to the side of the fuse to get the muffler to attach. There is a alot of material there, and it is double up....I was concerned...but pressed on. In addition I soaked the snot out of the balsa with thin CA and then put a coat of clear fingernail polish over it to fuel-proof it. After that I cut the heck out of the cowl to clear the muffler and went to the airfeild.
Flies like a champ with close to 100 flights (probably) and is one of my most dependable/fun aircraft I take to the feild. Good luck
....or...could be that your 91FX is a lemon...I had one and sent it back after several deadsticks and other concerns and they sent me a new one...The new one ran great after it had ran two tanks of 30% heli through it to finally get it to run straight. After that I switched back to 15% Byron and it goes smooth...no issues.
Hope this helps
Brett