Too much heat and/or too much acid destroy the thickening power of corn starch. If you want to know why the filling for lemon meringue thickened, but turned runny later -- that is ALWAYS the reason.
In Gummy's case, too much acid. She can make her pie more sour by using less sugar. For you other two ladies, too much heat. That's a matter of technique, not ingredients.
There's a "no fail" technique for making the lemon filling involving tempering.
Whisk your filling mixture off the heat in a saucepan, with 1/3 to 1/2 of the total water, cold. Meanwhile bring the remaining water to the boil When the water boils, whisk it into the mixture until fully combined. Put the mixture on medium-high heat, stirring slowly and continuously with a wooden or heat proof "rubber" spoon until it boils. Immediately, reduce heat to medium low and stir for exactly one minute, and remove from heat.
It's a very simple technique, but you must pay attention and respond to the temps -- especially when the mixture comes to the boil. If you answer the doorbell or phone, the pie will fail. Out of spite, if for no other reason. Inconsistency in the temps you set on your stove, and/or the way you handled the mix as it came to the boil are most likely the reasons your results are inconsistent.
The problem is certainly not the torch. In fact, a torch is the gentlest way to handle meringue on top of this type of filling. Using Swiss meringue to top an American lemon meringue didn't cause your filling to break. More, you certainly do not have to make a baked filling for a successful lemon meringue pie. You can make a good pie that way, surely; but it's a different pie.
You'll get less separation, less shrinkage, and less weeping if you pour the filling into the shell when it's still hot, and cover with meringue when the filling is still warm. If the weather's particularly humid, you may want to try sprinkling a little corn starch on top of the filling before covering it with meringue.
Hope this helps,
Edited by boar_d_laze - 10/3/10 at 10:12pm