It could possibly be the turn signal/wiper stalk (aka multifunction switch) - do you have any problems with the turn signals? When partially depressing the stalk for left or right turns do you get an electronic/metallic buzzing sound? When the intermittent are working, do they occasionally act erratically? 1 swipe...pause...3 swipes...1 swipe...2 swipes...etc? Do your sprayers work each & every time you press the endcap?
This is a common problem on older Xs and yes, ours are getting pretty old...it's $60 at Ford and an easy swap.
My 1997 Explorer XL just started showing problems with the windshield wipers. This problem popped up during last rain storm, if I have the wipers in intermittent setting, they stop at about mid travel and stay there unless I switch wiper control into constant ..ON... setting, in which case, they work fine. I dont know if there is a wiper control module, and if there is, where would I find it? I bought new wiper blades a couple months ago, was wondering if they might be cheap wipers that are sticking or hanging up on the widshield surface? Any ideas, need my wipers, its raining hard here in Cali. 
