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You may have 14 gage wire ran to your outlets, that is a little small for the 1500 watt load, especially if there are other things also being powered by that same circuit. do you know the amperage of your circuits ?
15 amp circuits aren't large enough for that heater, they will draw 13.6 for the heat portion only, then a fraction of an amp for the fan motor, so say about 14 amps, so if there are any loose connections or too small of a wire in that circuit or anything else is also running on that circuit it will overheat the outlet. It should be OK in the 20 amp circuit but needs to run by itself, this is probably a problem with your power or plugs rather than the heater as there isn't much that can happen to the heater that would make it draw more amperage than its rated for and still work.
You may want to have an electrician come in and evaluate it, he can also check the heater to see if it is over-amping ,