Heater will not stay running. I have one of your bigger units with wheels on it. I bought if

Question

I have one of your bigger units with wheels on it. I bought if about five years ago and recently it has a problem running. It will usually start off alright and run for about 15 minutes and then it will stop. After that I have to unplug in and then it will run for a few minutes again before it quits. Can you tell me what is wrong with this unit.