Dear David R,
I experienced a similiar problem in Sept 2007,when mine was still under warranty. I returned it to the servicing agent, Contrast Technologies Ltd in Co Durham (tel 01388812123) service@contrast-uk.net, who repaired it and said it was due to the wear on a magnetic strip of some kind. Wish you well.
Regards,
Ke. T
K.T, January 2009