White Knight dryer cutting out at random?

Have a White Knight CL731 condenser dryer - the trip switch for the heating element seems to cut out on occasion for no obvious reason. Have dismantled it recently to replace the belt when it snapped, cleaned the fluff out of the motor, the filter and condenser are both clean, trip unit has been replaced but it still cuts out periodically, on either heat setting. Trouble is it is random - may run all day on high without tripping, or may trip after 30 mins on low and 15 mins after resetting.
Any ideas?
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