Saab: 1986 saab 900s not charging, ammeter, hydrometer

Question
My 1986 saab 900s has lately not been holing its charge.  I have recently checked the battery with a hydrometer and it is fine.  My alternator has been replaced with a new one a few weeks ago.  However it still dies sometimes and leaves me hanging.
If anyone has had similar issues or know something about it if they could please let me know

Answer
Jonathan:
 Your problem can be caused by one of three possibilities:
a. battery dying  (apparently not)
b. Alternator defective (do you get >13.5 volts at the battery with the motor running?) If not, then the alternator is not putting out. Check belt tightness first.
c. Current drain with engine and accessories off: Possible; a sticking relay or switch stuck "on"? One way to test is to use an ammeter..if impractical, disconnect the battery cable between car usages..see if the battery then holds a charge. If so, then you'll need to check the electrical system, circuit by circuit.