I for one cannot help you; but having a very similar heater, I have exactly the same problem...its rather dented now, with a great many knuckle shaped dents where, with a lot of choice expletives I've given the blasted thing several good Lampings for cutting out - as you say - every 15 or 20 minutes.
If there is a way around this frustrating problem I will watching avidly for it to appear...
Or a recommendation for a similar product that doesn't have this rage inducing
safety feature.