oven keepscutting out :(

Hi, ive recently moved house and there was a electric oven already in the house when i moved in. im planning on getting a new kitchen in about 3 mothes times and dont really want to by a free standing oven. the oven in question is a indesit kwe1/g. The hobs work fine, but when you turn the oven on and turn the temprature up after about five minutes the oven completly cuts out and the light goes out in the oven , when you turn the oven off and back on again the light still stays off untill the oven has cooled down. i got a bloke out to look at it and he said it was more and likely the thermostat but he was charging £160 to change it and i cant warrent this for 3 monthes of use. so my husband found a thermostat on the internet and removed the old one and replaced it with the new one. when he turned the power back on it seemed to work, but then when we tried it today it still cuts out. when you set it to about 150 you can hear it click off about 100 degress and then it cuts off.

any help would be apprceiated

claire