Sound disappears for half a second (in pro tools)

Hi,
I’m using VEP5 in Protools 10 on a Mac pro (mac os 10.7).
I’m experiencing the following issue during playback: regularly the sound from all the instances of VEP5 disappears for half a second (sometimes more) and then returns. So for an instant I only hear my other tracks (audio tracks and other virtual instruments) before the sounds from VEP5 come back. It’s very disturbing.
To give some exemples of virtual instruments I use in VEP5 : Kontakt 5, Addictive drums, Mellotron Pro, B4 II.
I downloaded the last update of VEP5 and the last steinberg elicencer just to be sure – the problem remains.

Did somebody expirence the same kind of problem ?
Does anybody know where it can come from?

thank you!

Sacha