Display resolution sometimes wrong on power up

[Colin Law]

I have a strange situation with one of my machines. Running Jaunty
Desktop (new installation). Sometimes on power up the display goes to
800x600 instead of 1024x768.  If I then run xrandr it shows only
800x600 and 640x480.  If I logout and in again then it goes to
1024x768 and xrandr shows the full set of valid settings.  I have
Visual Effects off.

In case this is useful here is an extract from lshw (hopefully the correct bit)

     *-pci
          description: Host bridge
          product: VT8378 [KM400/A] Chipset Host Bridge
          vendor: VIA Technologies, Inc.
          physical id: 100
          bus info: pci at 0000:00:00.0
          version: 00
          width: 32 bits
          clock: 66MHz
          configuration: driver=agpgart-via latency=8 module=via_agp
        *-pci
             description: PCI bridge
             product: VT8237/VX700 PCI Bridge
             vendor: VIA Technologies, Inc.
             physical id: 1
             bus info: pci at 0000:00:01.0
             version: 00
             width: 32 bits
             clock: 66MHz
             capabilities: pci pm bus_master cap_list
           *-display UNCLAIMED
                description: VGA compatible controller
                product: KM400/KN400/P4M800 [S3 UniChrome]
                vendor: VIA Technologies, Inc.
                physical id: 0
                bus info: pci at 0000:01:00.0
                version: 01
                width: 32 bits
                clock: 66MHz
                capabilities: pm agp agp-2.0 bus_master cap_list
                configuration: latency=32 mingnt=2

All suggestions gratefully received

Colin