Lotus Notes R5 causes Navision 3.6 to run slowly

We are currently using Windows XP Pro (SP1), Navision 3.6 and Lotus Notes 3.6. On some PCs, Navision starts to run like a dog on a periodic basis[xx(] 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.

Sorry, I should have said that we are using Lotus Notes 5.0.11