1999 disco 2 engine shuts off

I have a 1999 Discovery 2 with 116,000 miles. It had overheated one day in November 06 and blew a hose. I had it repaired and then it all went to hell. The mechanic stated that the car was in need of a head gasket due to the missfire and the coolant system pressurizing too much. They fixed that and now I have a car that will not stay idle. While in the hands of the repair shop, the car was at idle then it just stopped. The check engine light was on and the car would not restart, it would sound like it fired but would shut down right away. After letting it sit for 10 minutes it would start again and remain on for 5 - 10 minutes and shut down. Mechanic stated I needed a new battery as a code shown "improper voltage to operate" put in a new battery now the alarm is acting funny, no lights just a beep of the horn without locking and still will only idle for about 5-10 min. So today we looked at it and the new battery was drained, jumped it and it started.
we pulled the engine and aux relays from the fuse panel and cleaned them and the car idled for about 1 hour and shut down and goes back to 5-10 min of idle.

so are the relays bad, is the computer shorted, is the alarm causing this, or what? The mechanic that did the heads is of no use at all. We are at our wits end since the car idles great, starts right up then it just shuts off.
PLEASE HELP, I SPENT 1500.00 FOR THE HEADS TO BE DONE AND IT WONT RUN!