Hi Dougal,
What revision is your A2000 motherboard? My 6.0 revision board (has a small 74ls373 riser board next to the chipmem) occasionally displayed strange artifacts when I only had 512K chipmem. When I did the motherboard mod to enable all memory as 1MB chipmem, all the artifacts disappeared and worked perfectly.
Artifacts like that usually point to faulty memory or Agnus, but you say yours works perfectly the rest of the time, so this one might be difficult to nail down. Try taking out your Agnus and reseating it or replace your OCS Agnus with an ECS one and try enable the full 1MB as chipmem.
NB: If you do decide to replace the Agnus, just make sure you get one that has the exact same pin-outs as your original one. There are many different versions of Agnuses (Agnii?) that are the same physically, but have different signals on the pins.
Cheers
