What colour is the exhaust smoke, is it black or blue/white would you say the running is erratic,

Some possible areas to check defective stop unit,
Restriction on induction system
Restriction in fuel pipe is it a banjo fitting onto the fuel filter feed pipe dirt can collect at this point.
Sticking valves , broken springs incorrect valve tip clearance
Defective injectors incorrect pump timing
Poor compression.
Hope there's some ideas for you to check out

Regards Titus