Fuel starvation 50% power – Ford Focus 2.0TDCi 2004

My car sometimes reduces power when under load such as accelerating up a hill by about 50% accompanied by a large amount of black smoke. The problem is immediately resolved by switching off the engine and re-starting. Then we are back to normal. N.B. This only seems to occur after a short journey followed by the engine being off and then re-started. The car had two fuel injectors fitted a year ago, also it has non-return valves fitted on the fuel line due to an earlier starting problem. Although it’s 18 years old the car has only done 80,000 miles and always been well serviced and maintained. Any suggestions for a solution to the problem would be most welcome. Cheers, Jim

Brand: Ford
Model: Focus
Year: 2004
Engine: 2.0TDCi