the cocking lever on my FWB300 pops open when I pull the trigger at least 30% of the time. Any thoughts?

Are you sure it's latching when you close it? There is a detent, and a locking lever on the cocking arm. Does it lock up solid?

Next: is your breech seal (in the compression chamber) sealing when closed? Tip: put a small (eg 1" x 1") toilet tissue on top the compression tube by the breech face and fire a pellet. Does it jump? If so that would indicate a poor seal and a new breech seal would need to be replaced.