sushihero

Hello everyone!
lately i have been experiencing a weird behavior of protools 9 + rme fireface 800.
While mixing and during playback, if i decide to switch a channel output between busses or channels (i have an analog summing device so it's quite often) and go from one output to another, the sound "locks" and keeps reproducing the sample (something like SBREEEEEEEEEE->forever). this stops when i switch the output again.
What this could be? i tried reinstalling drivers. same problem. It happens even with delays and reverb tails. if there is no sound while switching: no problems.
Thanks for the patience (my english is quite bad, so i hope i have myself explained clearly)!