Starting Problem 311cdi?

Hi. I am having a problem starting my 2000 Merc 311cdi from hot. Well actually it's getting worse now and will not start again even after the engine has run for a couple of minutes from cold! Glow plug circuit tested and checked out so checked the relay and ended up changing that thinking it could be the temp sensor on it. Still no joy. Starts first time from cold and will run fine all day, but forget and turn it off and I may as well come back the next day. Possibly worse on a hot day! Any ideas??? Thanks