Linux guest's clock freezes on a specific date and time

[broon]

Hi!

I have a ubuntu 64bits host ( Virtualbox v. 4.0.8 ) running on a server machine and I have a Ubuntu 12.04 server guest running on it. Every now and then the guest clock freezes on a specific date and time and I have to correct it. NTP can't fix this. I've attached a print-screen png that shows time going four seconds and getting back four seconds. It comes to normal after I correct the date and time. Important: I can't install guest additions. It says 'The headers for the current running kernel were not found', and 'Building the main Guest Additions module ...fail!'. But I've already installed the linux-headers-`uname -r`.
How could I fix that?

thanks in advance.