Take it from me, like I said in your past threads I manage with ambient temps in the 68 degree range. All its takes is tweaking and a little effort. Your best bet is to get some flexwatt to warm the cool side and run that off of a dimmer. You said you had a good central A/C so you shouldn't have to tweak it too much once you get it stabilized. $30 will correct your problem.
Switching to a tank will not correct the issue your having. It will probably get worst since there is more air space inside a tank for the heat to get dispersed in.
4 inch strip of flexwatt running across the cool side, controlled by a dimmer; thats what I would do.