i thought the pfaff machines have built in - even feed- which means you do not need a separate walking foot= you just set the machine for even feed.
anyway- if the foot you are trying to use is not made for your machine that could be the problem- i would contact the pfaff company or the dealer and ask for the correct foot-
the one you are using could be the cause of the screw coming out too- vibrating-or even turning it as it stitches.
i know we all like to save money- but sometimes the more expensive product---made for our machine is the only way to go- using the incorrect only costs more in the long run- from replacing- or repairing damage caused from trying to save a few bucks....i about choked when the walking foot for my machine cost $79...when i knew my friend had just bought a different one for $12---but then it cost her over $300 to repair her machine- from using the foot she bought- and i decided i got the deal.