Garbage on the display with Radeon AGP mode enabled and Lubuntu 18.04

Hello all,

I have an old HP laptop which I happily use daily, but is getting slow with some web pages and I would like to enable the AGP graphic mode.

However, when I enable the AGP mode of my radeon driver with radeon.agpmode=4, the display shows some artifacts and garbage as depicted in the image at the link.

https://postimg.cc/cvztKRFX

Note that the garbage seems to "move" even though I'm not doing anything and also disappears when I move the mouse over the area, then returning after a while.

I tried many if not all of the switches available with the radeon driver, but none eliminated the garbage.

Do you know what might be the cause of this problem?

See some characteristics of the laptop I have.

gs@gn-lap:~$ inxi -xxx -G
Graphics: Card: Advanced Micro Devices [AMD/ATI] RV250/M9 GL [Mobility FireGL 9000/Radeon 9000]
bus-ID: 01:00.0 chip-ID: 1002:4c66
Display Server: x11 (X.Org 1.19.6 ) drivers: ati,radeon (unloaded: modesetting,fbdev,vesa)
Resolution: 1600x900@60.00hz
OpenGL: renderer: llvmpipe (LLVM 10.0.0, 128 bits)
version: 3.3 Mesa 20.0.8 (compat-v: 3.1) Direct Render: Yes

gs@gn-lap:~$ inxi -xxx -S
System: Host: gn-lap Kernel: 5.4.0-81-lowlatency i686 bits: 32 gcc: 7.5.0
Desktop: LXDE (Openbox 3.6.1) info: lxpanel dm: lightdm Distro: Ubuntu 18.04.5 LTS

gs@gn-lap:~$ inxi -xxx -C
CPU: Single core Intel Pentium M (-UP-) arch: Dothan rev.6 cache: 2048 KB
flags: (pae sse sse2) bmips: 3389 speed/min/max: 1700/600/1700 MHz

Thank you very much