Yes, assuming it was running OK for awhile and this behavior just started, there's a good chance your rear bearing is starting to fail. Small bits of metal flake off the inner and outer races and find their way to the glow plug.
A plug fouled with bits of metal will start and run, but generally the motor will quit when you remove the battery, or it may run until you get to about 1/4 - 1/3 throttle, at which point it just quits.
Changing the plug can seem to make things better for awhile till another bit of metal finds its way to the plug.
Remove the motor, disassemble it, and inspect your rear bearing closely. Also inspect the piston skirt for scratches, the crown of the piston for dents or pits, and the inside of the cylinder head for dents or pits.
I recently replaced the bearings as well as the piston and ring in one of my OS 50s after it flamed out in mid air. The bearing was indeed bad, and under a microscope, I saw bits of stainless steel embedded in the piston skirt (it was a stainless steel/ceramic ball bearing).
I caught the problem in time and that OS 50 SX-H runs as good as it did when it was new and just getting broken in.
It's a good idea to wash out the innards of your muffler if the bearing has gone bad, as bits of metal that make it through the motor can get stuck there, to be sucked back into the motor at a later date, putting out the fire...
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