I pulled out the 20D today, and looked at the card in the camera, it was a old Hitachi 6GB Type II Microdrive. Talk about old.
I had a no name (Literally) 4GB card laying around, so after charging the battery, cleaning the sensor and a general checkup, as well as clearing all the settings, and then setting it to raw, I gave it a try.
I hooked up my Canon TC-80N3 and shot off about 40 images at ISO 400, 1/50 sec in manual mode with manual focus and 3 sec intervals. I could see it was taking about 2 seconds for the light to go out, and then, I realized it was in raw, so the files were larger.
Then I switched to large jpeg, and shot another 40 in a row with no issue.
I then put a faster spare Sandisk 16 GB 60mb/sec card in and shot off 50 images at 1/sec with no delays.
Finally, I put the old generic card back in and shot off 50 images on one sec intervals. The write light just barely flickered between shots, but it was obvious that it could keep going forever that way with a slow card.
Its somewhat a puzzle as to why your camera is stopping, but I would clear it back to the original settings and try that. There are some settings that could cause issues, so its best to test it with the default settings.
I like the old camera, it works well. The small LCD is really the only drawback. The sensor does have a few white pixels, but that's pretty normal for a old sensor, and NR in Lightroom removes them in any event.