I never use IS on a tripod, I've seen the results on that, the IS will make tiny circles hunting around.( ten to thirty second night exposures) Your 5D and 500 are in a whole different class of IQ than the 300/7D.
It could also be vibration from the wind, I'm not going to get one of those heavy Berlebach tripods either, aluminum was heavy enough.
link to 135 w/1.4x at distance showing heat and wind but still "crisp". Granted my PS sucks but I don't see this with the 300/1.4 at distance. The distance is ~1.5 miles
http://kevingoto.smugmug.com/Sports/...tXhp9&lb=1&s=O