To understand Achilles problems, you have to understand the mechanics of how it developed in the first place. Legally blond is spot on that the problem may be from alternating higher heeled shoes and lower heeled shoes.... tightening it and stretching it over and over again, which is going to cause microtears, scar tissue, and inflammation. Furthermore, you're doing your 'most stressful' runs in the lower heeled shoes-- the stress of forceful running/pushoff combined with the stress on the Achilles from being in the lower heeled shoe is too much!
To take this a step further-- what are you wearing for your street shoes? If there's a sharp contrast between your street shoes (~amount of heel, stiffness) and your running shoes, that could definitely cause problems.
Additionally, you talk about getting sore going through sand. My guess is your shoe is 'sticking' to the ground as your heel/foot lifts up and off the ground.... this is going to 'tug' on your Achilles. This is exactly why I suggest the barefoot running cause you eliminate this 'tugging' action by your shoes. This is especially problematic if your shoes are loose.
You literally have to find grass ASAP when your Achilles gets the slightest bit sore. If you wait too long, scar tissue will develop and it gets trickier to treat (although I still fully believe in the therapeutic benefits of barefoot running, even for the most severe problems). You need to get your Achilles healthy... there is no point in racing and possibly knocking yourself out of running for a while.
Should add contracting forcefully.
Totally agree. I don't think you have to make a conscious effort to change your mechanics. You simply learn to be the most efficient and more 'gentle' with less on your feet. However, it's hard to realize and understand this if you're only doing hard runs in the shoes. I mentioned this in my first post-- you gotta 'lift' off the ground, not push. Land closer underneath your body, rather than foot stretched out on the heel (~overstriding).