Dragging Brakes (over time) 2001 Ford Focus LX

My brakes drag (I'm assuming), and the dragging worsens after about 10-15 minutes. The car also pulls to the left, especially when applying pressure to the brakes.

When I first start my car and begin to drive, the drag is extremely subtle, almost unnoticable.
But after driving about 10 or 15 minutes, the dragging worsens to an almost undrivable situation.

Things I have noticed are, in cool weather, or when driving at night, the brake pedal sinks down much further before grabbing, and the dragging is less intense and takes longer to intensify.
But if the weather is warm, or my car has been exposed to sunlight, the brake pedal barely goes down before it grabs, and the brakes will usually begin dragging within 2 minutes.

We've replaced the brake hoses, replaced the left caliper, and replaced the brake pads. The hoses have been bled numerous times.
Another thing, when the hoses were bled, we noticed the brake fluid is extremely thick, almost like syrup. The fluid we use is DOT 3.

A friend suggested cleaning the master cylinder.
A mechanic suggested (since he had no clue what was wrong) replacing everything. I'm thinking it's something simple, just nobody seems to know what it is.

Any ideas? If your suggestion ends up fixing the problem, expect a donation. :D