I don't run electric rigs, but this sounds like a faulty termination (electrical connection), possibly between the controller and the element. It just seems strange that it would do fine one smoke, then have trouble on the next with little ambient temp difference. If it's heating at all, the element is fine...they either work or they don't.
I'd check wiring and maybe the controller function if you can. Put a volt meter on the terminals leading to the element and get a reading when it is heating. Should be about 120VAC, if much lower, there could be a resistor built into the controller, but not likely.
If using an extension cord, make sure it is of the proper AWG size for the amount of amperage/watts the smoker is rated for. This is a common mistake...too light of a cord, and it won't carry the current required by the appliance, so the appliance will not operate at it's full potential.
I won't go into recommending a possible replacement smoker, as there are so many makes and types to consider. I like the rigs I have, and use them for particular smokes as they may do me a better job than another. Or, one may be just a standby or backup if I just can't fit everything into one rig that day. Or, maybe I want a charcoal fired smoke for that really great flavor and color to the meats. Really cold and ugly weather? No problem...I'll just go the easy route and fire up a gasser.
Good luck!
Eric