TV-out problem with ATI Radeon 9550
 

I have have been experiencing trouble with TV out from an ATI Radeon 9550 card on my MythTV box. Here is what happens (I am including details of hardware etc. further down):

Using the ati kernel module everything works perfectly on my PC monitor.

When I install the ATI proprietary driver I get choppy video both when watching recordings and "live TV". This happens irrespective of whether I direct output to the PC monitor, to the TV or to both. I have tried playing with the different options available in the proprietary driver, but nothing has worked so far.

Here are some details on my system:

Pentium III 1 GHz, 382 Mb RAM.
Motherboard: Gigabyte GA-6CXC7
ATI Radeon 9550 128 Mb AGP

I run Debian Sarge with a custom compiled kernel 2.6.20.

Incideltally, I first had a Pentium III 600 MHz processor with 256 Mb RAM. Even this worked fine with the ati kernel module and output to the PC monitor. Assuming the problem was connected to hardware I switched to a 1 GHz processor with 382 Mb RAM, but the problems with the proprietary dirver persisted.

Is there anything obvious that I have overlooked?

Thanks!