Formerly Known as Newbie
Join Date: Mar 2002
Location: Helsinki, Finland
Your comp battery may be low too. My wireless Sigma acted all weird, but the display was still readable so I thought it cannot be the computer's batteries. After eliminating pretty much everything else I changed the comp battery and that fixed it. At that point I also realised how dim the display had gone as the battery kept getting weaker.
Some tools to help you determine where the signal is lost:
I seem to remember Sigma wireless transmitters make an audible "click" when wheel magnet passes the sensor (a reed switch, I believe). Mine also had a test function, a blinking led in the handlebar mount, to indicate signal reception. If the signal is OK all the way from the magnet to the mount, then it's either your computer batteries, or a bad connection between the comp and the mount. Or interference, as Michel suggested. Not sure how to test for that, except eliminating everything else.