Can only tell you our expereince: seeing a bad winter coming, we bought 4 space heaters for extra-cold days and in case the central heat would fail. We checked ratings in "the leading consumer research magazine" and picked a highly-rated model: UL approved, well-known brand. We're used them: each connected to a different circuit, not run for long periods, thermosats set at a reasonable temp so they cycle on-and-off........so far, 2 have failed (one just doesn't work anymore, thermostat MELTED on the other). We got them at less than list price - paid $75 each - expected a little more. Don't buy Honeywell !!
No, don't think it's your outlet: too much power could damage the heaters, but low voltage/amps wouldn't. Any signs of problems with the outlet (melted/distorted, discolored?).. I just think that you got junk ones, like we did.