brake caliper locks up

i have a 92 Ford crown vic. When i drive it, it seems fine, but after a while the brake caliper just locks up on the front driver side. Does anyone know why this might be happening??

Bad flex lines
Bad caliper

I'm not sure a bad flex line would cause it, but I would go with a bad caliper. When were the pads changed last? Was the bolts that hold the caliper on cleaned with a wire brush and new white grease put on it? How was the caliper pushed back when new pads were put on? Most of the new and especially the non-PD versions use a plastic piston, which can be canted, resulting in not moving back when the brake pressure is released. I would look at that first.

The flex lines can cause the problems. They will deteriorate on the inside and a little piece of the rubber will drop down and cause a "one way valve". This does not allow the fluid to return quickly enough causing hydraulic pressure to stay against the piston in the caliper.