Please see KB69211, I believe this is a known issue with HW02.
What diagnostics have you done already?
Have you managed to pin-point the cause of the reboots? Is it the cluster or one of the hotfixes.
If you haven't tried already (or if it's even possible) roll-back and implement each change one at a time and test the pptp connection.
EDIT: They're quick around here on replies he beat me by a few minutes.
Thanks for the helpful reply.
The update described in KB69211 rectified the problem.
Thank you very much!