Xp SP2 IBM think Centre PC

intermittent fault:

when looking at directories on the network, or on local c: including when in the office and when diialled in through VPN the screen refreshes as if you were pressing F5 every couple of seconds (its not the screen at fault its the computer refershing the foldres list) dont know why its doing this.

I can normally find answers just through google but its not  the easiet problem to describe precisely.
CPU etc is fine when this happens, it only happens a couple of times per week, no particular directories being used, cant find any consistency to it. Cant logon as someone else to see if it happens as it only happens a couple of times per week.

Any thoughts?