Canon EF 70-300mm IS USM with issues.................

[mr.mike]

I have a very nice Canon EF 70-300mm IS USM lens that's acting peculiar. Seems to work on my Canon EOS 5D Classic, but doesn't work with my Canon EOS 20D. When placed on the 20D the IS function jumps a bit and then goes dead with a 001 error message. Sometimes it'll fire, but no image appears on the screen. When placed back on my 5D, the IS jumps a bit, then locks up focuses and takes a great photo. Anyone have an idea what the problem is? I'm also interested in knowing the reason for the IS jitter.
Looking forward to hearing from you,
Mike