1999 Volvo V70 stall out

Hi, My car is firing up fine, but it cranks, fires, then stalls immediately. Sometimes if I push the pedal to the floor and hold it down while I turn the car over it will finally stay running. Other times if I just leave the car to sit for 20-30 mins. It will fire up and stay running. But on 2 occasions after the stall out happened the next day my battery was totally dead. When I discovered the battery was dead by trying to start the car, there was a clicking sound coming from somewhere in the engine like something trying to turn over even though the key was not in the ignition at this point. The sound sped up and didn't stop until I reinserted the key into the ignition switch.
So the 1st time I took the car in they put it on the computer for diagnostic codes and got none, but they said it was due for a electronic throttle module upgrade, and maybe this would fix the problem, so they did this and it didn't fix it. The second time I took it in they said one of the battery cables was getting too hot and this was causing the stall so they replaced it, and the car stalled the next day. My friend who is a mechanic suggested the throttle body could be sticking and another mechanic suggested it could be a bad relay switch (I don't even know what that is).
I just read through some of the other questions posted here with similar stall out issues and wanted to add that I did have the same problem 6 years ago, and it was diagnosed as needing a new MFI module, antenna and immobilizer code, new keys etc. It didn't dawn on me until now that the car's behavior is very similar to what it was doing then. I hope it isn't the same problem -$600 plus buckaroo's to fix. Ugh.
Can you help me identify the problem before I am stranded once again?
Thank you!