My Leyland Daf 45 registered 1992 drives excellent except after about 40 miles (less in hot weather) it starts to stall. Cuts out when I brake or slow down. The further I travel, the more frequent this happens until after approx 60 miles, I struggle to keep it ticking over. Had various parts replaced. Has anyone any idea what the problem is? Help please.

On diesel engines that will not restart or stall, after they are warmed up, it's often a sign of a tired fuel pump. As they heat soak, inside pump head clearances increase and it will no longer develop sufficient fuel pressure at cranking speed. While it's running, fuel helps cool the pump by carrying the heat away. Therefore, I would check the fuel pressure, when the problem starts to develop. It goes without saying that you need to regularly change all under hood and under frame fuel filters.