Just driving along at 70 looked at the speedo and it suddenly dropped down I think to 30 and back up to 79 while cruise control on? Any ideas? Current stable:
2006 BMW M6
2007 4.2 V8 S/C FF
Gone:
2002 4.4 V8 FF

I know the speedo is fed from the rear left wheel..... there is a reluctor ring.....

Though when mine was iffy on my TD6, the speedo just read 0, so the car thought the other 3 wheels were spinning when you drove it......

Not same symptom, but might help with diagnosis.

 2011 Range Rover Vogue SE 4.4 TDV8

Previous cars :
2003 Range Rover Vogue TD6
1999 Discovery Td5 ES
1995 BMW M5 3.8 6 speed
1992 Range Rover 3.9 Efi Vogue
1992 BMW M5 3.8
1988 BMW 735i SE
1989 Ford Sierra XR4x4 2.9i
1981 Ford Fiesta Supersport

Thanks hope it was a blip for now! Current stable:
2006 BMW M6
2007 4.2 V8 S/C FF
Gone:
2002 4.4 V8 FF