I have been having the same problem with my 01 550.
What I did
Checked it with a buzz test, GP test, and contribution test. All passed.
Then it died on me going down the road, with a 20 hour old cps in it, so ruled that out. Acted as though the timing was goofy, lots of blue smoke, no power, then blow a cloud of smoke, and run ok.... hmmmmm
Tested low side fuel pressure, had 60 psi. Eliminated that possibility.... or did I?
Hit the key, and listened to the lift pump, you could hear it cavitating. Sooooo.
Replaced the stock fuel pickup line with a 3/8 diesel rated hose, eliminating the stock push lock connectors on the tank and the pump. This reduced the cavitation, but did not stop it.
Next step, pulled the fuel pickup module out of the tank. The pickup tube was intact, as was the return line, so, I went into the mixer.
On the bottom of the mixer, is a pre-strainer. sort of a "rock filter" if you will. then, inside that mixer ( the white plastic piece ) there are two more wire mesh filters, somewhat finer than the strainer. Both of these were partially plugged.
Upon cleaning the mixer body and filters, I began reassembly. When I thought about WHY the return line was plumbed into the mixer body, which would help introduce entrained air into the suction side of the lift pump. So, a 14" chunk of hose later, the return is routed away from the mixer, the port where the return used to go is sealed off, and the mixer is now used strictly for fuel pickup.
Reassembled the unit, put my 3/8" fuel hose back on, attaching it to both the tank pickup line and the intake port on the pump, carefully tightened the hose clamps, and turned it on.
It took about 20 seconds to purge all of the air I had introduced to the system, and now.... NO MORE CAVITATION... yay n stuff.
Easier starting, smoother running, all in all, a win/win situation.