What video card are you using? Going from a std consumer type card to a fire gl T-2 was the best $200 we ever spent on hardware. Even if you have a good card, its worth trying different drivers, if even only to eliminate them as a cause.
Monitor task manager. Check your memory useage. If "Commit charge total">("physical memory total"-"physical memory available") by more that
20-30 meg then you are probably using virtual memory and need additional ram.
Turn off the system-drawing options for smooth dynamic motion and show box contents while dragging, unless you have to have them at the time. Ensure that your document properties image quality has not been messed with (ie. you turned it all the way up so that you could select a vertex that was being a PITA, but forgot to return it to a reasonable value).
If any of your drawing views are shaded, check to see if any of the visual properties have been altered. Optical properties other than default are resource hogs. Check to see that no materials or textures have been added unless required. I think someone has a macro that resets all visual properties and assigns random colors to each components that might be useful here.
I can't fully understand this one, but, fonts other than standard windows/sw ones seem to cause me lots of problems even when TT.
Create a new configuration for your assembly and place a fresh view of it on a new sheet. Supress components in groups to see if its an issue with a single sub-assembly or component. I have an actuall full model of 2 1/2" diameter 10" length perforated tubing, 3/16" holes on 3/8" centers, staggered pattern made from sheet metal. It will bring any system to its knees.
Not better? Try unsupressing all components and supressing all mates (provided you do not need dynamic motion or require further updating). Fix all components.
Good luck!