Pan & Zoom suddenly slow & jerky in model space

Autocad 2008 has been running smoothly for a long time. This morning on trying to use the programme, working in model space has become almost impossible as the pan and zoom functions have become very slow and move in short distances before a pause and then moves again. I've tried restoring my pc to a previous restore point as I had a windows update last night, but it won't restore. If I access the model through a viewport the pan and zoom functions are normal. Anybody with any ideas please ? Thanks.

Did your view accidently get changed from plan?

Thanks Rob but I have just sorted it ! Not an autocad problem but a hardware issue. I use a wireless mouse and have been swapping it, with the wireless receiver, between my laptop and pc. It's normally plugged into the back of the pc but last night I plugged it into a usb hub on my desk (got fed up grovelling on the floor to get to my pc). It worked ok in everything except pan and zoom but in desparation I plugged it back into the pc and hey presto - no problem. I had considered it might be the problem before but as it workd fine for pan and zoom through a view port I thought it can't be the mouse ! Technology's great when it works but very frustrating when it doesn't. Thanks for your response.

That is funny because as I was reading your post I was thinking it was a hardware issue until you said it worked fine through the viewport.