The only time I have heard of constant MKIV rebooting is when it is trying to read a copied map DVD. Test it with an original map disk to see if that improves it. If not, try it without any map disk at all.

If it needs a new laser, there are several posts here about replacing the laser.

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SOLD
2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input