Ovens coming on unrequested!

I have a new to me but second hand Stoves gas hob, electic oven. Irregularly when turned off, the small oven comes on….maybe once a week. Fidling with the switch does not usually turn it off (have to turn off at the wall). I assumed a thermostat problem and was heading in that directon… BUT now the other (larger fan oven) has the same problem which seems odd to say the least. Im a) struggling to find a mechanic who will do gs/electric and b) when I eventually do I’d like to know what the problem is as my epereince is that this leads to clear service…. Any ideas anyone about what on earth this ia about??