Usually, this is caused by the bolt not quite catching on the sear. The benchmark triggers can have some wobble in them and this can cause the trigger to not be as precise as it needs to be and can exaggerate the issue of the sear/bolt slippage. I suspect that you will find that the sear tip and/or bolt lip are rounded a tiny bit. If this is the case, you should look at replacing the sear if it is rounded. The bolt may be ok, but you will need to evaluate it.
The cause could also be associated with a leak in the on-off area. Does the gun leak out the front at all when you fire and hold the trigger in?
The Classic valve is fast, but it can't recharge fast enough to be reactive and maintain usable velocity.
Except for the Automag in front, its usually the man behind the equipment that counts.