That is extrodinarily unusual for a Beretta 92.  They are big and have a trigger that takes some getting used to, but they are normally rock solid reliable.  Until the locking block breaks, but that is the subject for a whole 'nuther thread. 

I wonder if the recoil spring is correct.  Could the guy you bought it from have accidentally subsituted the original recoil spring for a spring from a different pistol like a 92 compact?  Could he have stored it with the slide locked back for long periods of time?  A factory recoil spring should be about 5" to 5.25" long.  Some aftermarket springs can be 1/2" longer than that.  If yours is less than 5", get a new one.
I would also consider replacing the extactor spring.  If the gun was stored for a long time with a round in the chamber, that could weaken the extractor spring too.