Motors quit because they aren't getting fuel/air in the right mixture, or the glow plug is shutting down.
Fuel related causes can be too rich or too lean on the needle settings. Check all hoses for leaks, check the fit of the stoppers in the tanks, check the tightness of the muffler to the crank case.
Glow plug problems can be an old plug -- perhaps damaged by cranking the motor over with a hydraulic lock once or twice. The glow plug could be loose in the head, bits of metal could be flaking off the rear bearing (the ugly OS SX-H/Hyper 50 problem) and finding their way to your plug, or if you used RTV to seal your muffler to header joint, bits of that could be flaking off and being sucked back into the combustion chamber, finding their way to the plug. I've also seen a small, crystalline deposit form on the glow plug coil after many flights. It is a small, hard ball that usually forms between the weld on the plug body and the first "turn" of the coil. It's usually easily picked off without damaging anything and the plug will run fine again.
If it's bits of metal or RTV on the plug, the motor will restart, but die either immediately upon removing the glow battery, or as you throttle up to about 1/3rd throttle. You may even get into a hover before the thing dies. If the bearing is going bad, you can appear to have a bunch of plugs that are bad. I've gone through maybe 5 plugs in an afternoon, thinking I'd just gotten a bad batch of OS plugs. I then discovered that the rear bearing was going bad. I cleaned the plugs in an ultrasonic cleaner, the bits of metal were washed off, the plugs were fine. With the RTV contaminated plugs, once the goo gets on them, they are toast. Washing doesn't help.
Other causes could be that your backplate might be loose, or the head could be loose.
Water in your fuel can also cause erratic engine behavior. Fuel slurps up water from the atmosphere if your jug isn't tightly sealed between uses. The motor can run just great and stop without warning. Or it will idle poorly or transition poorly. Bad fuel can cause all sorts of crappy behavior.