99 forrester slow fuel fill

I am a certified mechanic at a shop, and nobody here can figure out this problem, so hopefully someone on here can help. We have a 99 forrester 2.5L and if you try to fill the tank at the gas station, you are lucky to get $1.00 worth of gas before it hits the auto shutoff. if you try barely squeezing the nozzle, you MIGHT get $2.00 before the shutoff. We have figured out that it is due to the tank not venting the air as it is replaced with fuel quickly enough. Subaru dealers are as stumped as we are. We have installed a brand new factory fuel tank, all new sending units/aux pump/fuel pump etc. and bypassed the rollover valve. even replaced the filler neck twice brand new from dealer. If anyone can shed any light on this we will be forevermore in your debt. thanks.

Mike