Does the striker move freely? If it is sticking that would cause the stalls
Are you using a remote line? If you have a flow restriction the remote will serve as a small buffer, once you deplete the line pressure the system recovers after you pause and hit the reset.
Have you tried a different magazine? Any time you have stalls at groups of 8 I get concerned that there is a tight spot, or two, on the chain. This causes the bolt to require extra pressure to overcome and most times if you are following proper reset procedure you are dropping the magazine and this releases the tension allowing the bolt to reset.