Screen corruption on R300, glxgears turned into modern art

There are a number of bug reports on screen corruption with either older Radeon cards or specifically for R300 but these do not seem to match the problem I'm seeing so here goes:

On an older machine - Thinkpad T42p, i.e. RV350/M10 GL [Mobility FireGL T2], Pentium M - I've noticed screen corruption on 'more recent' (i.e. 5.x) Linux kernels while things work as intended on older kernels. A few examples show the extent of the corruption for those programs which I can get to run:

chromium renders garbage:

text corruption in mate-terminal:

glxgears shows modern art:

Firefox does not run when using webrender, it fails to initialise a GL context ([GFX1-]: Failed GL context creation for WebRender: 0). Font rendering is totally messed up, things go from mostly readable to garbage to semi-readable to almost normal to garbage to ... depending on what happens on the screen. Sometimes black noise fills parts of the screen, then disappears, then returns elsewhere. In short, things are a mess.

I've tried the workarounds which have been mentioned for 'similar' problems - i.e. iommu=pt, iommu=soft, AMD_DEBUG=nodcc etc - to no avail, these are probably applicable to more recent hardware.

Some data on the affected system:

Thinkpad T42p (Pentium M 1.8GHz, 2GB) X.Org X Server 1.20.9 Chipset: "ATI FireGL Mobility T2 (M10)" (ChipID = 0x4e54)

[   134.907] (II) Module glx: vendor="X.Org Foundation"
[   134.907]    compiled for 1.20.9, module version = 1.0.0

[   134.908] (II) Module radeon: vendor="X.Org Foundation"
[   134.908]    compiled for 1.20.4, module version = 19.1.0

Using EXA, DRI2

No errors in Xorg log

Using Xmonad, not running a composer.