I've run into a few problems with PPTP on PIX over the years.
I got one sorted today so figured I'd share the love.
We switched from a direct ethernet connection to being behind DSL router and NATing everything.
For some reason PPTP stopped and gave the error...
GRE request discarded from my.ip.add.ress to outside:x.x.x.x
The TCP 1723 part was fine but the GRE was now broken. After some debugging and testing it seems the change from direct to NATted we need the
fixup protocol pptp 1723
command. Once this was in it all worked fine again.