Land Rover 90 Wipers Wheelbox / Wormgear problem

Hi
I have a '84 90 and am having a battle with the windscreen wipers, thus I have changed the wormgear, wheelboxes and spline adapters. So pretty much everything bar the motor
My issue is the wipers work fine with the blades off the windscreen but as soon as they are in contact with the screen the wipers don't behave. I can see the gear inside the wheelbox stuttering forwards and backwards as it slips on the worm gear. I have tried tightening down the two side of wheelbox where the wormgear enters and leaves the wheelbox which did help a little.
Any ideas, suggestions, tips gratefully received

disconnect the motor and pull on the wiper rack it should pull quite easily with wipers on the window, if thats ok either wiper motor is weak or the drive wheel has worn teeth