Hey clive, I replaced the rubbers with new original items at great expense and it made no difference whatsoever, so I cleaned one of the old ones and put it back in drivers door, no difference !!! so I guess it is the motors, I'm going to do the same as chris and give milners a shout !!!!
As Chris has said in detail, I think the problem at 21 years plus is that no one thing is the cause for sluggish operation, it's the overall effect of many.
The mechanism and the runners in the frame, as new, will all be tight, no play anywhere and with a mech that relies on so many factors, particularly cantilever arms, toothed drives, and pivots, that little bit of wear (a lot in some cases) and misalignment may be enough to overpower the motors.
I can't think of an explanation for mine binding when wet. You'd think water would at least act as a lubricant, but it seems to be the opposite. I can only think that the felts in the rubbers expands or something similar, causing the extra drag.
Anyway, as Cris also says, for £60 it's a one-stop solution, hopefully. I'll report my findings if I can manage to get some.
At the moment, in the dry, mine take about 10 seconds to drop and maybe 15 to rise, but sounds like a huge struggle for the motor on the way up.
We'll see...