Nav Unit Problem

[apacheclay]

I am seeming to have an issue with my factory touchscreen navigation unit in my 2004 Tahoe. The unit works great, but it seems to have an issue like it is getting hot, and the navigation stops working. I have been on a few short trips and I set my destination, the navigation works great but it seems that 30-40 minutes into my trip I get the message that the DVD is damaged or dirty, and navigation stops. I thought this might be an issue with the disc so I brought a spare along on my last trip and when I had the problem, I changed discs, but no luck. When I get to my destination and the truck is off (nav unit cooling off) everything works fine again. I see that there is a fan on the side of the unit, should this fan run all the time or does it come on when it's hot? If anybody has any ideas on what could be causing the problem it would be greatly appreciated.