I had the same problem with my readings except my eha was static as well. I am assuming you are using the eha cable that mb makes for diagnostics. The sears multimeter the positive(red) should be on the #2 port and negative (black) should be on the #3 port of the x11. Key on engine off should read 70%, if not then chack the cd manual it will tell you the error. If this is not fluctuating then you are in an open loop. The EHA as steve says is the key to adjust the system to your eha number and the duty cycle should match, although the O2 sensor should effect the eha reading.
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)