2001 Renault Megane losing power

I have a 2001 megane 1.4 16v that loses power once warmed up. No warning lights on and no fault in memory. When let to cool for a few mins with engine off all power is restored and then loses it again after a few mins of driving. There is no misfire but when fault occurs the engine speed seems low approx 500 to 600 rpm. Has anyone any advise on how to deal with this fault?

thanks