This problem has been observed under Windows.
You probably have some other program running in the background. While PVCR plays a battle, it constantly polls the keyboard. Therefore, Windows thinks PVCR is idle, and it would be better to give the CPU time to the background process. Windows doesn't see that PVCR also updates the display constantly.
The known-good fix is to stop the background process. Maybe you also manage to fix it by playing with the DOS-box property settings. The quick workaround is to press a key or fiddle with the mouse so Windows thinks you're talking to PVCR.
This has been observed with the program CPUCool, but probably things like Seti@Home are good candidates for this problem, too.
