Hi,
Sounds like a memory issue - you may need more... most likely Magix may not be freeing up what it uses. Unfortunately that is common with audio programs. They use lots of memory and CPU cycles for certain tasks and do not release them when they're done. Make sure you have large buffer settings in Magix and then download a memory cleaning utility like FreeMem. There are many free ones out there.

That should be your first step, if that doesn't work consider contacting Magix or reply back here and we'll try a few other things.
Justin