Battery getting old?
How old is your battery? If it's older than three years, I'd certainly suspect it is just getting old. When vehicles sit without being run, as yours probably was while it was being worked on, an older battery will tend to deteriorate rather quickly, due to sulphation of the plates. I'm thinking your battery might have just reached the end of it's useful life. If you are going to be parking your vehicle for more than a few days at a time, it would be a good idea to get one of the low current trickle chargers that are available at places like Harbor Freight, and keep it connected whenever the car is parked. I use them on all my rigs that aren't driven regularly, and they seem to really help things out, especially on the newer vehicles that have current drain on the battery at all times due to the various microprocessors, etc that are always on. My sister's caddie will kill the battery in less than two weeks of sitting!
'82 300D/4.3L V6