heats up fine and to correct temp, but wont regulate? indesit kd6c8ew ?

indesit kd6c8ew.
main oven seems to heat up o.k and always gets up to right temperature quickly, but more often than not when left for 15 min or so the temperature will start to fall, the fan will stay running and the indicator light stays off as if its still at correct temp, but there is no heat. if then i turn the control down past min temp. and back up to cooking temp. it will always kick back in and rise to temp. quickly again. - and the process repeats. since it reads correct temp. am not sure if it would be the thermostat needing replacement- or is there some kind of seperate regulator? -I have looked on parts lists for this oven and cant see any part i it could be other than a thermostat or a component that is listed as a cut -in and has no description. can anyone tell me what part i need??