I can confirm that I (and another composer running VEP) have also experienced issues like this running VEP AU instances in Logic Pro 9.1.1 under OSX10.4.6 running the latest VEP on a slave Mac Pro over gigabit.
I wasn't able to track down the exact cause and the behaviour was sporadic, in fact the other composer who I've been helping with VEP has just called me with this issue and a 1 hour deadline. What a pain!
I just thought I'd chime in and say that Tonus is not alone with this type of behaviour.