I have an intertherm heat unit in my moble home. When temp falls below set temp on tstat, unit comes on. As desired temp is reached, unit continues to run. Will not shut off until main power is cut off and unit is allowed to cool for a couple of minutes. If you immediately turn unit back on, unit will continue to run, but after cool down period if you turn back on, unit will wait unitl tstat calls for heat again before coming back on.

Sounds like your contactor switch is bad and needs changed out. Don't attempt to fix this yourself if you don't know what you are doing. Best to call a repairman and have it properly tested and fixed.