Sounds to me as if the pistons are not travelling immediately, which would fit with "stops when it wants to, eventually". With the brake off, there shoudl be a gap between the pad and the disc (piston in). Slide a piece of paper between pad and disc. Apply brake. A properly funcioning brake will stop you pulling the paper out instantly, a sluggish brake will stop you pulling it out after a second or two. Is yours instant or sluggish? If it's sluggish I would suggest a problem with the calipers, in that the pistons are sticking for some reason. Strip down, clean, renew seals, rebuild, retest