After upgrade to 12.04 got Nouveau invalid parameter error fails to start graphical greeter, thrown into text login

Upgraded via update-manager from 11.10,
On restart got from tail /var/log/Xorg.0.log

 23.997] (II) NOUVEAU(0): NVEnterVT is called.
[ 24.661] (EE) NOUVEAU(0): failed to set mode: Invalid argument
Fatal server error:
[ 24.661] failed to create screen resources

The monitor briefly displayed 2 messages
[drm:drm_crtc_helper_set_config] *ERROR* failed to set mode on [CRTC:6]
then gave a text login prompt.

Tried renaming, and also copying /etc/X11/xorg.conf from xorg.conf.default still with same effect
Tried nomodeset but got a very slow, tiled, stripey desktop, unusable

Any other options worth a try?

Any way to start X from the text mode after login?

Note: can run from liveCD 12.04 with no problem, tried this before upgrading, so
Is there somewhere else it is picking up the invalid argument to pass on? (For example a hidden config file in /home?)

Is this likely to be a bug?

Note: have to run with option nolapic because of another bug.

excerpts from other logs
[drm] nouveau 0000:01:00.0: Detected an NV10 generation card (0x018100a2)

NV18 [GeForce4 MX 440 AGP 8x] (rev a2)
 Kernel driver in use: nouveau
 Kernel modules: nouveau, nvidiafb