My Ex did that to the extreme, frequently accusing me of cheating on her. If she called me at work and I missed her call because I was away from my desk, she would accuse me of having sex with someone at work. Often I caught her going through my phone, pda, web browser history, even my car. I suspect that it was a mental illness. I tried every way that I could think of to get her help. Through all of that I was indeed faithful to her, until the the divorce.
If you find yourself compelled to engage in that kind of behavior, PLEASE GET PROFESSIONAL HELP. It could save your relationship and possibly your life.