undesirable movement of the simulation box in NVT ensemble

Dear Steve

Thank you for your reply.
Either I use a wall or do not use that, the box moves up surprisingly in NVT ensemble. I did monitor it using VMD, and it moves up in both cases. However, there is no movement in NVE and NPT ensembles. If I attach spring to the region near the wall, would the box be fixed?

Thank you very much again

Saeed Doagou-R