My 1999 slk merecedes convertible top will not go down sometimes or come up.

This symptom almost always points to a mechanical issue, not an electrical one. The SLK hardtop is notorious for needing to be lubricated every couple of years. Here is a link with directions and photos:

http://www.benzworld.org/forums/r170-slk-class/1426014-lubing-vario-roof-pics-1-krytox.html

Sticking joints are the most common culprit to intermittent raising and lowering problems. Once those have been lubricated and that has been checked off the list, then look into electrical issues.

Mark Andrews

It can also be a abs sensor gone but the roof will not operate at al, nsf (English) is the one that control the roof amongst other things, I changed mine and it works ok now, part number is 2025402317 L, you will find this on the sensor cable, very easy to change but there are two type, at least for the '99 model one with esp and one without, mine is the one with out so check if you have esp, if you do the above number will be wrong.
vicppl