USB 2.0 falling back to full-speed

I've recently had to change my MB to a M2A-VM (AMD 690G/ATI SB600 
chipset), since then I have problems with the USB2.0.

The syslog most of the times shows at boot the ehci_hcd kernel driver 
(the 2.0 driver) trying to assign some address to the 2.0 hub, and the 
hub refusing it or failing with some I/O error. The driver tries 4 
different addresses, then falls back to ohci_hcd (the 1.1 driver) and 
works fine but at low speed. The whole procedure takes maybe 1 minute, 
for this time any peripheral connected to the port will not be detected.

Google suggested hw problems (e.g. cables), so I installed Windows and 
did some test, looks like it is working fine at high speed (I used an 
utility similar to usbview to check the driver tree). I'll do some more 
testing to exclude the hw problem.

Note that since 2.6.26 the kernel displays the "unable to enumerate USB 
device on port x" at every boot, but according to Google it is 
relatively common and does not harm functionality, so I believe my 
problem is something else.

If anybody has some suggestion I'll appreciate.

raf