Ubuntu – Clock stops ticking when inactive, causing drift

The clock applet drifts in time.
Clock is set to "synchronize with internet …" so it is correct at startup, but then if I stay inactive for some time, may be 5 min as well as 1 hour, the clock stops ticking.
If I start to be active again, then the clock applet moves again, but the time is now late.

And it is not only the applet that is wrong, but the whole system date, because when I run date in a terminal, the time is also wrong.

Clarification :
Sorry, may be my question was not clear. Here is my bug report to ubuntu :

Expected Behaviour :
Clock-applet displays the correct time,

Observed Behaviour :
Displayed time is drifting

How to reproduce :
If I get away from my computer for some times, the time displayed by the clock applet drifts. But the date command also show the wrong time. Moreover, sleep interval also get wrong. To debug this, I tested the following script :

#!/bin/bash
while [[ true ]]
do
    date >> clocktest.log
    hwclock >> clocktest.log
    sleep 300
done

Must be run as root because of hwclock.
Il launched it :

./clocktest.sh &

and got away from my computer

Here is the output log :

1 mardi 17 août 2010, 12:42:12 (UTC+0200)
2 mar. 17 août 2010 12:42:13 CEST -0.346882 secondes
3 mardi 17 août 2010, 12:47:13 (UTC+0200)
4 mar. 17 août 2010 12:57:13 CEST -0.080965 secondes
5 mardi 17 août 2010, 12:52:13 (UTC+0200)
6 mar. 17 août 2010 13:02:14 CEST -1.002776 secondes
7 mardi 17 août 2010, 12:57:18 (UTC+0200)
8 mar. 17 août 2010 13:07:18 CEST -0.063633 secondes
9 mardi 17 août 2010, 13:02:18 (UTC+0200)
10 mar. 17 août 2010 13:12:19 CEST -0.361501 secondes
11 mardi 17 août 2010, 13:07:19 (UTC+0200)
12 mar. 17 août 2010 13:17:20 CEST -0.987434 secondes

Line 1 and 2 show the first time through the loop.
Line 3 and 4 show the bug : while date (and sleep) thinks 5 minutes have elapsed, hwclock shows that 15 minutes have elapsed.

Line 5 to 12 shows normal behaviour, except now date is late by 10 minutes.
Behaviour is normal because I was back at my desk using the computer.

Having clock applet displaying the wrong time is one thing, but having the whole system time wrong (since sleep gets confused too) is a major bug.

Hardware :
It is a fujitsu siemens amilo xi 2550 notebook.
It was working fine with ubuntu 8.04