Are you saying the heater has it's own timer? If so, it sounds like the timer is reverting back to some sort of default setting. Read the manual and see if there is some way to over ride this. There must be a "clear function" process of some sort.
For starters, do not unplug the heater inbetween times you attempt to get it to work. Lack of power to the unit may cause it to revert to default.
I have an expensive Tstat on my radiant cove heaters and it does that whenever there is a power outage or when the unit is shut off.
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Current snakes:
0.0.1 Gonyosoma oxycephala - Java locale (green)
1.2 Gonyosoma oxycephala - Jave local (green)
2.2 Gonyosoma janseni - Seleyar locale (all black)
1.2 Gonyosoma janseni - Celebes locale (Black & Tan)