roaming user sesion

hi
it sims im ready geting lot of problem hope one can help me out on this one i got a domain winserver enterpris where all user are on roaming profile with folder redirected to the server for a wile im getting this problem with most user every thing suddenly disaper for thier destop and give an error you no longer conected to server but if you clik on the taskbar on the little pc icon wer it says offline an press sync everything will come back without even loging of.in ad all profile is set to sesion never expire but whats strange is when everything goes away the lan is still connected you can even brows the net.

plz help