hi everyone, I'm new to these boards but i was wondering if anyone could help me?

Recently i have purchased a year 99 space wagon. everything has been fine until last few weeks when the car was taking a couple of attempts to start but when it would start it would seem to lose power without cutting out whilst the car was stationary.
Then last week the car would not start so we jumped started it and that was fine. the next day the battery was totally dead so we replaced the battery. Everything was fine but then around 6 hours later the new battery was dead. Our neighbour who is a mechanic came to have a look nad siad that because the doors wer not flush the drivers door light was staying on which drained the battery. The lights were disconnected and left overnight. The next day the car started but lost power and cut out straight away, though the radio would work so we knew the battery wasn't to blame.

We had a couple of suggestions such as the alternator was faulty, so we called an auto electrician who said that if the car was starting but then cutting out and the radio was working it wouldn't need an auto electrician to look at this. So we called an tuning company who came out and they said it was the body throttle adn would cost around £1200 to get this fixed. He said he had done something has a tempory measure and at the moment the car works fine e.g starting and not losing power. Does anyone know what it maybe or has experienced similar kind of problems and what was the fault, as i don't fancy paying out £1200 on an item when imay not be needed