AEG 520b-d oven temperature problem?

AEG 520b-d oven temperature problem
erratic results when cooking using conventional setting, not fan setting
wife complains she can cook two cakes, using same mix, one okay, other sunk in middle
have replaced bottom element, top one works okay as grill, and changed thermostat, no difference
any suggestions before i buy a new oven
have used a temperature probe on multimeter, and apart from fact temperature 20 degrees lower than dial, it stays consistent, with about 5-10 degree switching range