Ah, with the last description it became clear: ignition or fuel problem!

Ditch the plugs ASAP: when the engine starts rocking and it won't boost it's dropping onto 3 cyllinders! Interestingly that's what my first cossie did before it melted a piston! Check plugs and injector: maybe one of the injectors is blocked!

If it does have the syndrome you shouldn't sell it: you should push it in the river...