@acer I copied acer's practice of putting the restart command at the beginning of the document and that helped. I haven't had the problem since, but it is an intermittent problem and I am not sure it is gone for good. My gut tells me that the restart command isn't enough.

@Carl Love I deleted all of the Ps and typed shift+p in their place i.e. I made sure that all of my Ps were Ps instead of capital rhos. It didn't make a difference. If you note in the screenshot the last diff statement works properly and has both an italic P and a straight P.

I took out the restart command and tried using "assuming conds" instead of the assume command. Although this worked, when I removed both the restart and the "assuming conds" I couldn't reproduce the problem anymore.

Thanks everyone for your help. I have learned helpful things and hopefully one of those things permanently fixes my problem.

@Thomas Richard I used a screenshot because I can't always reproduce the results. Sometimes it works and sometimes it doesn't. I am using Maple 16.