Pam:
to make sure you have isolated the interval that is causing the trouble, simply eliminate that interval blast (keep the cardio before hand and substitute the side taps move with the move from another interval or sequential power kicks swapping legs half way through) the next couple of times you do Imax 2, then assess how you feel after wards. If there is absolutely no discomfort trhen you have successfully isolated the cuprit and you should substituite another move for that one forever after. There's no shame in that. I never do the sumos blast because the arms movement hurts my shoulder too much so I always do sequential power kicks and believe me, it's definitely up at the anaerobic threshold.
However, if you should still be experiencing discomfort, then that interval is not the culprit and you should consider staying away from step for a week or two just to rest the foot and ankle. You really don't want to mess with your achilles.... I get twinges every now and then and it's a signal i have been doing too much step, very reptitive movements, so i go to hi/lo and kickbox for a few days and that does the trick. It's good to cross train anyway....
Also, how long have you had the sneakers? The cushioning may be shot, if you've had them longer than 6 months and ou always do high impact, this could also be the culprit. Achilles ache is also for me a yearly sign to shop for new shoes....
Clare