if you look at bruno's wiki at bmwe34.net, there are instructions as to how to install the retroactive fuel pump check valve kit. i got mine from the dealer about 3-4 years ago for under $10, might be higher now. i'm not saying that this is what is causing your problem, but you might want to do it at some point (even if that means on a new fuel pump).
that being said....where did you measure your fuel pressure, at the rail, or at the pump? ymmv, but if your fuel filter is fresh, there's something i like to do that lubes fuel pumps (tends to quiet them down a good deal for a long time), USUALLY bumps the pressure up at least 1 or 2 psi if the unit is marginal or poor, unsticks sticky fuel floats, and strips the shiat out of carbon on your pistons. do this at your own risk, as i don't know what state your equipment is in. pour a quart of marvel mystery into 10 (preferably 5) gallons of fuel and run till your light comes on. you might want to wait until an oil change to do this, though.
ps...take the end of the vacuum hose off the fpr after running and see if it reeks of pure gasoline. that'll end the speculation quick.
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