Cruise control works intermittantly--but rarely on 1999 Volkswagen Passat

Have had this problem for 2 yrs, mostly just don't bother, but would be nice on trips to have the cc work. The odd thing is, it works occasionally for 4-10 miles or so, then cuts out by itself. But mostly, it will not work at all.
I have had the cruise control electronic unit replaced, which did not help. I've been leery of throwing more good money after bad. Suggestions?

2 answers
Do both of your brake lights work? If not fix that and see how your cruise behaves.
Other than that their are a number of possibilities , from the multi function switch , brake light switch , clutch switch , control unit and the wiring in between. The servo is vacuum operated so more possibilities there.

This really needs to be diagnosed properly , per the factory manual to avoid wasting more money.
I have an 02 Passat....had it in to the dealership 7 times for a very similar would cut out on its own...and after several attempts, it was some type of brake switch that needed to be replaced. This also fixed an acceleration problem I had as well....apparently my VW dealership had to contact the Regional VW rep to figure this out.....