Sorry but your post is a bit confusing... you think the rotors got too hot, and one of the symptoms was early wear on the pads....thats correct?
You said you changed rotors and pads...did this yourself? if so did you use the right tools to compress the pistons? if not compressed properly theres a chance you can deform or crack them. This will surely could make a piston get stuck in the bore, but most surely will also create a leak.
Now...slotted rotors do wear pads faster, and they do tend to CRACK at the slots. Also, when a rotor gets too hot, it could warp.
Remove the pads and rotors, have the rotors checked for warping and cracked spots, and if needed, do a resurface.
Don't use pure metallic pads, use more friendly semi-metallic ones, will brake better for everyday use.
Change the brake fluid, your 99 stang probably still has the old fluid in there, and a good chance it has a good amount of water.
Have the shop..or yourself, inspect the pistons and rings. Replacing the rings is a simple process, just make sure the piston gets straight into the bore.
I do love the look and added braking of slotted rotors, but using a high quality brand is a MUST. There are cheap ones that basically are stock rotors that have been drilled...those are CRAP.
Even the quality ones DO CRACK, i had brembos and willwoods and eventually both cracked at the slots, but they lasted a lot and got serious track time.
Rob Hernandez, AFM co-Founder.