904 shift kit problems

I'm with Bob on putting a blocker rod under the accumulator so the piston doesn't slam back and forth. I doubt that's your problem but it is a potential problem as the piston may eventually break if left like it it. Any rigid 1/2" diameter round material can be used to make a blocker rod.

Did the kit also have you install a restrictor in the main case where the high clutch is fed?

I have had issues using B&M kits in the past and now only use Trans-go kits. Been using Trans-go for about 12 yrs. with no problems.