A-5 velocity dropoff

Alright, so I looked thru a couple topics about CO2 but it didnt catch anything on this specific problem. Ive had an A-5 all stock except an all american barrel, for about a yr now. ive got a 20 oz tank and the first 100 paintballs i shoot are all very consistent, but then i seem to get a dropoff in ball velocity. Now im not sure if this has something to do with the gun or just the fact that im using CO2. someone suggested the low pressure kit, but i dont want to spend $120.

so help me out guys. any ideas?

In addendum, if you're having shoot-down with that few shots and you aren't blazing away with your a-5, then chances are you're either playing in cold weather and using a lot of liquid CO2 which empties your tank more readily (OR, as the case may be, you're shooting hot for the first 100 rounds and then the tank actually has room for the CO2 to expand in) or that there is some sort of an issue with your marker being starved for air. Look up shoot-down under the search list and you should find a decent amount of topics.