I have same macro and same problem, but i'm not running i'm killing around same spot.
The big prob is that i loose autofollow even if i log the toon on the 3rd comp... so no boxing at all. Still i just press the follow macro and the toon is back on track...
I guess I have less "autofollow" problems then u guys.
It looks like the loading of the area, for eg. gfx., mobs, etc. effect the rate of loosing autofollow.
When I hunt in the same area, and all gfx load activity is finished, I usually don't loose autofollow.
When I run long distance, it's mostly when my PC lags due to new gfx. load, that I loose the autofollow.
I find that my boxed toons lose (not loose btw ;) ) autofollow. More often than not when running long distances but they can lose it whilst fighting in small areas. I believe this happens simply due to lag.
lag and small bumps in terrain, or rocks, corners, small doorways, multiple characters on AF and getting stuck in a pile... anything can interrupt AF very easily, and not just on boxed characters... i frequently have people go afk for a drink or something similar and follow me while we are hunting, its incredibly easy to get them stuck through no fault of your own