I Purchased one of the n scale S4's and liked it so much I ordered another.  I was running it on my test layout and it ran perfectly for an entire 3 hour operating session.  The next time I ran them (in DCC mode and they have the same address as before, both engines are #15) and the more I ran it the more trouble I began having with 'dead spots'  I recleaned the tracks and other engines run with no problem, but not the S4's.  In fact they have become so spotty on the power pickup that I just can't use them at all.  I cleaned the wheels and even took the truck frames off and cleaned behind the wheels.  there wasn't much there to clean as very little came off on the alcohol soaked paper towel I was using a bit of to reach behind the wheels.  I think I need to take the driver axles out and directly clean the wipers, but really, after the extremely minor dark spots I got with much cleaning, I am not sure this is the problem.

If it is, how do I get the wheels out of the truck after the bottom is off?  I dont want parts flying all over.

thanks for your help in advance.