I fought a highly intermittent P0410 in my C230 Kompressor a couple of years ago over a period of six months.
It ended up being a clogged EGR feed line, I think there was a thread on it... have you seen it?
I found it....
http://www.peachparts.com/shopforum/tech-help/193953-99-slk230-code-po410.html
Don't know if its your problem but it would appear that the car you are working on is prone to clog the EGR line since MB apparently replaced the line at least once. BTW no need to replace the hard line, its easily cleanable.