And do using PCQ ?
There is an old bug in mikrotik at radius client which causes some big problems like yours but mikrotik hardly defends against it and did not confirm that .
The bug is not in pptp server . it is in mikrotik radius client . look at radius client timeouts . you will see a lot of timeouts there . when radius client freezes , the interfaces hide , routing will have problem , users could not authenticate , sometime snmp breaks , and many many more problems . i am following this problem in this topic http://forum.mikrotik.com/t/routeros-radius-client-timeout-pppoe-pcq/34479/1