My money is still on the fluid being old and full of water. If the brakes are good on first use, and degrade with repeated hard use, but then recover, I'm struggling to see how it's the lines. Possibly a pad issue, but I'd have thought that were that the case, the recovery wouldn't happen. Bottle of dot 4 will only be a few quid - although if you're not completely comfortable doing it, find a friend who is (where are you?) - and replace the fluid completely. You'll be able to see when the new is coming through, it'll be a different colour... Sent from my HTC Desire S using Tapatalk 2