passenger door window

can anyone help with regards to a little problem I have with the passneger door on my 97 5 door. the window will go up and down with the motor so I think thats ok but as it comes down it starts to fall out of true in the runners, I'm assuming ( dangerous that) that I've got to replace the winder mechanism.
How difficult is this?
I'm ok with most work but have always shied away from meesing around inside the doors.

If it is not clucking on moving up or down (when the regulator teeth start jumping on the motor gear)it might just be sticking in the guides,I would try some silicone spray in the channels,to see if it helps-otherwise if you change the regulator,it is not too bad a job (door card off),just mind some of the metal has sharp edges

How do you mean, runs out of true? It may be a problem with the long grippy thing that holds the bottom of the window edge? They do rot