Could be any of several things:
-Active bass? Might have a dying battery.
-Instrument cable from bass to amp may be acting up? (Try another cable)
-Amp may have a switching jack; it it's got a preamp out/power amp in loop, the switching jack is notorious for getting a little corrosion in the contact causing volume dropouts. Explained more thoroughly here, with a procedure to test :http://music-electronics-forum.com/t28549/
-Potentiometer in either the bass or the amp may have gotten a little old and corroded and could stand a little treatment with Deoxit to remove the corrosion.
-Problems with the speaker
-Other problem internal to the amp itself.