7422GX clock skipping seconds

Hello. I'm puzzled by the strange behavior of my 7422GX's system clock. About 2 months ago I noticed it was skipping some seconds and thus giving me the wrong time whenever it was on. It was not a matter of whole hours as it would happen because of a badly set time zone, but I would put the clock on time early in the morning and after some hours of working it would be 10 minutes back or so.

By the time, it had a strange network behavior and I found traces of a rootkit, so I backed up, erased all the partitions, formatted and reinstalled Windows XP Pro. Set up SP2, installed NOD32 and few other things including spybot S&D and Mozilla Firefox+Thunderbird.

The clock kept losing seconds, though. It didn't happen with the computer off, so I rule out a battery issue. The puzzling thing comes here:

Whenever I started the Task Manager, the CPU Usage History chart went at the same pace as the system clock: skipping seconds!!! Instead of a regular rythm, the squares appeared every, say, 1 sec, 1 sec, 2 sec, 1 sec, 2 sec, 2 sec, 3 sec, 1 sec ... regardless of the CPU usage (which most of the time is less than 20%).

There's no change on the system's performance. Most programs and games work OK, but the system clock loses about 20-30 minutes each day.

I had an empty HD lying around, which I formatted and installed Vista RC1 onto, as well as Mozilla Firefox. It's been working for a week and today I noticed just the same problem: It's lost nearly 20 minutes.

In short: My computer's clock is skipping seconds and it shows also in the Task Manager CPU Usage History chart, which advances at a non-regular pace. It happens regardless of the operating system (XP or Vista) or the system load (usually below 20%). The PC keeps the correct time while turned off and the performance is unaffected. The skips happen at random and unrelated to the programs in use or services loaded.

Looking for someone with a similar problem or a clue.