PHP unexpected Session expiry

[#1]

My PHP sessions keep expiring randomly, sometimes after only a few seconds. I'm wondering if there is some problem with shared storage for session data. Anyone know anything about this?
    Mike

[#2]

I seem to recall something like this happening recently and the cause was I believe the /tmp filesystem running out of space (which is where sessions are stored).
Best to raise a ticket and get it checked out.

[#3]

Thanks Peter. That sounds very plausible. I'll raise a ticket.
    Mike