erratic network latency jitter problem

Hi

For some time now I have had the problem with my laptop that the network latency of my computer is constantly varying (called jitter). The effects are visible if I ping a server, the output is that every second ping response has a time of about 1000ms, while the others responses have a normal time of about 20-500ms. I have tested from other networks and the problem persists. I have also tested it with other computers on the same networks, and then there is no problem. I have talked this over with other, more experienced linux people than me, and no one has yet come up with an idea of why and how to fix it.

By accident I noticed that if the hardisk is working, as in full throttle work, the ping response times drops to the normal 20-40 ms, as soon as the harddisk stops working the response times experiences jitter again.

The machine is a thinkpad t60, with intel core duo, and a sata disk (run by the scsi system, not libsata). The kernel is a homemade 2.6.17.8 smp with preemtpive locking and big kernel lock. (the problem also existed on a previous kernel I used, 2.6.16.9)

The first thing of course is actually trying to identify what it is that causes this erratic behaviour. So the question is, does anybody have any ideas what could be wrong or what I can look at-for, to go about actually fixing this?

regards

tom