You could have a VERY weak battery. Maybe a dead cell. If you have a multimeter, check the battery whilenot running. It should read around 13 volts. If it is 11 or less a cell is dead. Replace the battery. If you want, most auto parts places will test the battery and alternator. I know Advance will, sure others will also. Somtimes a battery will look okay on the meter, but will fail when a load is applied. To test the alternator, check the battery with the car running. You should see about 13 to 14 volts. If it is less, alternator is not keeping the battery charged A 12 volt battery has actually 13.2 volts.
1995 325I, window tint, K&N, debaffled airbox, BMW #104 wheels, Nighthawk low beams, smoked side markers, smoked corners, smoked taillights, M3 stlye heated mirrors, various interior bits, Bosal cat-back.