Sounds like it could be a bead leak between the rim and the tire....often they'll act like that, and the fix-a-flat won't help much.
Easiest way I've found to find the leak is pull the wheel off and spritz it really good with Windex or equivalent and then wait to see where the bubbles grow. Very slow leaks tend to "foam" by growing a small cloud of tiny bubbles, while faster leaks tend to blow a bit bigger bubbles (1/4" diameter). A bead leak will grow tiny bubbles, almost like fuzz, right between the rim and the tire - just spritz a bunch of windex on a horizontal wheel at the intersection between wheel and rubber and sit back and watch. A leak in the tread can be found by rolling the wheel slowly along the ground whilst spritzing it with the Windex and watching for bubbling.
Once you find the leaks, you can determine if they're a bead leak or something caused by a puncture. If it's a puncture, then you can use a flat fix kit which has a rasp for cleaning out the hole, and a probe which inserts a puncture seal, which is an adhesive coated cord pushed into the hole with the probe. All pretty simple and they work great - just follow the directions.
If it's a bead leak, then you should take it to a tire place, have them pull the tire off the rim and then clean the rim bead with a rotary wire brush, goop it up with rim sealer and put the tire back on and re-balance it. Bead leaks can be a pain, so have someone fix it right.
I agree that fix-a-flat stuff can cause MASSIVE imbalance when used in a tire. The stuff pools at the bottom when injected and can sometimes gel there and stay put (because you ALWAYS take a few minutes to get it in there, then collect all your stuff, put the kids and the dog back in the car, get back out to check for anything left, get back in, make sure everybody's ready and THEN take off - during that time, the stuff has pooled at the bottom of the tire and hardened and acts like another wheel weight, but NOT where you want it!)
Gordon