[TheRamJam]

Have a look at the pins to see if there are any slight marks or grooves in them from wear in tear. If there is then its likely that is what's causing the problem. Replace the pins if needed.

With the pads out pump the lever a bit till you can see the pistons and give them a good cleaning with brake cleaner. Again check for any scoring our gouges. grease up and push back in. Hopefully this should sort the problem.