this is hosting related issue, we had several clients with this issue but always it was fixed by host before we catched and debuged all what it can be(takes time since we are not hosting company)
Best would be to contact your host, ask them what could it be, if they find solution post here, if not also post here or contact me over icq 169397168 and gimme your ftp and tp login info so we can check inside out.
Thnx