Anna,
70% sure you are looking at an intermittent connection at either the MP transducer end or the DPU end. If you have a screw terminal type DPU, then now I'd be 90% sure you have a loose connection at the DPU.
Check each of the connections for the MP transducer, terminal block J3 connections 9, 10, 11, and 12. Give each wire a small tug to ensure security then use a small jeweler's screwdriver to snug up the terminal screws, not too tight, just snug.
You can see my website for more good to know VM1000 information at
http://vmsupport.weebly.com/
Happy Flying,
Reggie