This could definitely be tricky to troubleshoot. A few questions come to mind that might help me get started:

When you say difficulty repainting, do you mean as it relates to the laggy behavior, or are there other repainting issues like ghosting or artifacts?

How does the lag act - does it seem like a slideshow - the whole screen updates at once but the updates are too far apart? or is it more like part of the screen updates, then another part of the screen updates, then another part of the screen updates, until everything is updated?

If it starts lagging after a few minutes - does the amount of time before it starts to lag seem to be fairly consistent, or is it pretty variable?

Once it starts lagging, does it seem to get worse the longer it runs, or does it get to a certain level of lag and sort of stay there?

Have you seen it start happening while you're in different zones, or have you pretty much been in the same place when it happened?