wipers acting wierd

My 1994 Cherokee's windshield wipers started acting strange. The other day it was raining i turned them on to the hi position, and they were going extremly sluggish, and the wipers wouldn't go all the way down. I figured i can see if its the motor by hiting the button for the windshield wiper fluid and sure enough the wipers were running normally.

I took the wipers off and looked at the motor everything seemed fine. With the wipers off everything works normal. Without the wipers on it runs fine an at its proper speeds when i change it from intermintent to lo to high. WIth the wipers on it runs slugglishly and sometimes sticks.

Is it something with motor wearing out? or what else can it be?

You lost me on the second portion of your explination?

I'd be interested in hearing some ideas on this myself. My '89 does exactly the same thing. I replaced the wiper motor with a new one and there was no change.