My 2000 model gearbox keeps dis-engaging once its hot, I've serviced it twice and after about a week it keeps happening, I jst bought this gearbox recently and I can't seem to get a reasonable explanation for this...what do I do cos its the 2nd time I'm changing gearbox in a year.

Trac is an expert in this, please send it to the Mercedes Benz Thread, it will be looked into for resolution by the experts.
Meanwhile, it sounds like low fluid presence in your transmission. Probably some clandestine seepage somewhere, when last did you change the fluid with the right spec, check it up to see the quality and texture...do a visual inspection underneath the car to observe for anything weird (leakage).
Another thing, get it scanned, and read the codes that come up, this will really help you a great deal!