cpu at 100% utilization

running an old dos-based 16 bit application in a dos window on nt server 4.0; when the application is open but idle task manager shows cpu usage at 100% when the application is active cpu usage drops dramatically and then increases back to 100% whenthe application returns to an idle state; this causes a very sluggish repsonse to all other requests made on the server; even unlocking screen server takes an excessive amount of time; what may be causing this problem?