The terminal output often goes missing - the space is there where lines should be but only black background shows. Usually seems to be the first 3-4 lines at the top of the terminal screen, but isn't consistent. Hitting Ctrl-L to refresh the output, sometimes fixes, but often the refresh just misses other lines. Moving the window reliably redraws it.
The problem occurs with both xterm and rxvt, so suggests it's a painting issue rather than the terminal itself.
Running Ubuntu 11.10, but I recall problem appeared (albeit much less often) with 11.04.
X is using the radeon driver with an HIS ATI Radeon HD 5550 PCI express card.
thanks in advance