Glow plug is a great place to start! I have two XXXNT's. One, I have a Picco .12 which came with it, and the other a HPI/Nova Rossi .12. Both engines run great, but that little rubber thing (stinger)coming out of the tuned pipe sometimes slides into the pipe blocking the fuel tubing which provide's the pressure to your tank! or the fuel tubing could be pushed too far in, and butting into the stinger. This problem could be yours as well. Yes, sometimes it is frustrating, but I only dis-assemble motor if all fails! Another suggestion, do you have two needle's? maybe a little richer on your low end will help you get to the top end! Good luck Let us know!
Kevin