I've got the same issue, having stripped it down the problem is due to the contacts of the forward/reverse switch being tarnished/burnt. I've removed the switch and taken it apart and cleaned it but it's still not perfect (intermittantly fails in reverse).
If you can remove the 4 screws that hold the case together and get to the switch, a squirt of wd40 may be worth a try. swapping the switch is a case of desoldering 5 contacts. I've not tried sourcing one yet although it does look like a standard switch that should be obtainable.
RichardH
April 2010