Bit of a follow up- I got a 7D in December, so haven't used the 24-105 on the T2i since.

I have had 0 errors with the lens on the 7D, hot or cold , fast (much faster that on T2i) shooting and stills.

So (shrug) no idea .. and not concerned anymore

I don't have a timer/intervalometer for the 7D so still using T2i and ML for playing around at night, I'll post again to this thread if issue revisits me.