go to your local leccy wholesalers and ask them for a can of switch/contact cleaner -Isopropol I think it's called or summat like that - you want
'the good stuff' ..... pop the cover off your slip ring assembly and give it a good dose -- slew her round a few times and give it a bit more --
rinse and repeat 

... see how that goes
edit
just remembered the trade name of some stuff i used to use a long time ago --
Inhibisol -- was the bollaux ... dare say it was
'nasty stuff' and probably not available any more -- worked bloody well though