I have a Viking Mower, I have some problems with the vario drive, I move the lever to a faster walking pace but it does not increase speed, if anything it goes slower or doesn't move.. the manufacturer has attempted to sort this problem out by replacing gearbox etc etc, but the fault is still there, and now they are refusing to mend this mower that's under warranty, any one know what can be the problem I'm having "HELP"