Zanussi ZCE 620 X oven keeps knocking itself off whilst cooking?

Zanussi ZCE 620 X oven keeps knocking itself off whilst cooking. I have just fitted a new thermostat unit as it had blown and the oven had been working fine for a few days. Whilst cooking something for example a chicken, say for 2 hours, the oven will heat to the required temperature and then about every 10-15 mins it knocks off for anything from 30secs to 1.5mins always comes back on and repeats the cycle. I have used a circuit tester on every single connection and everything is working. Everything else on the cooker is working, hob, grill, oven light, oven fan, but these go off when the oven knocks itself off. The clock and timer which is on manual also work perfectly. This cooker/oven does not have a cooling fan.