This kind of problem in the AT area can be difficult to diagnose and sometimes you may get conflicting advice, if you see a decent podiatrist/physio they should be able to suggest some long term stretches, with attention to the whole leg not just the achilles/foot area. There are various treatments to try from acupuncture to massage, ultrasound and of course the stretched... there are many to try, it really should be a long term plan though and probably good to stretch before,after and even on days when you're not running. Footwear, as already suggested, could be an additional area to look at.