Espace 2005 starting problem / reduced engine power

Hello, recently my Renault Espace 3.0 (diesel) has a problem to start the engine and sometimes also get a problem with reduced engine power. Problem with starting that after pressing start/stop button engine is trying to start (so battery is ok), but it doesn't start. I just hear that the battery tries to start it but without success. After pumping diesel to the engine (with small black rubber pump) it eventually starts. So it seems that somehow the diesel returns back to the fuel tank. Another problem is with loosing engine power. Sometimes happens, that the speed reduces so significantly that the car barely get 50km/h. I was already several times in renault service and paid them over 600 euros (totally) but they didn't find anything. I suppose they don't perform any deep analysis of the car - the diagnostic doesn't show anything and the car itself doesn't shows any problems on the dashboard so they are not able to check anything without any indication. Any suggestions what can be cause of the problems? Both problems (starting and reduced power) are random. Sometimes they occur after 1h parking, sometimes after a week of using the car. Together with those problem also the average diesel consumption raises by 1.5-2L/100km