CPU usage problem with Lotus Notes R5 & Navision

We are currently using Windows XP Pro (SP1), Navision 3.6 and Lotus Notes 5.0.11. On some PCs, Navision starts to run like a dog on a periodic basis. I believe it is due to Lotus Notes running in the background. What happens is Navision takes a long while to redraw each screen. Task Manager says there is 100% CPU usage, and NLNOTES.EXE is taking 99% of the CPU. Strangely enough, if you minimise, then maximise Navision, the problem goes away (for a while). Is anyone else who uses Notes experiencing this problem? More importantly, does any one know of a fix? Cheers.