If you are facing power issues - you are running low on fuel - meaning the injection pump and injectors are not fuelling your engine enough, You should see some smoke on full pedal acceleration and the engine should very strongly accelerate in lower rpms. If its not then you have fuelling issues.
I would go over the whole engine starting from valve clearance check, timing belt check, removing nozzles and checking breaking pressure and spray pattern - readusting the pump timing with dial gauge, checking the Cold start device on pump for operation and finally checking internal pump pressure.
I think that someone in the past has reduced the fuelling in your pump, hence the no smoke, no power situation - it will give you great economy though