3500 Watt 220

Hello, I have a question that I am hoping someone on this board can answer! I am having some repeated issues with an electric coffee roaster (basically a rotisserie oven). I have a 220 V requirement to run this machine, (wattage with multimeter has read between 210 to 220 depending on time of day, probe placement) and it powers a set of 3500 watt heaters. The temperature is user controlled through a rheostat, and this (from my understanding) is tied into a SSR inside the machine. I have replaced my SSR once, as it failed (reason unknown) and essentially "shut off" keeping juice from running to the heaters. I have lost heat again, and can only assume its the SSR again. My question today is focused on whether the SSR is overly stressed as a result of low/fluxuating electricity from the 220 source?