Going back to the original post I think the first thing I would have done is a compression test on all four cylinders also check that the coils are within operating tolerances and that goes for the plugs as well.. I have had a duff plug and that was a new one so its not impossible to buy new parts and for them to be duff..the next thing I would check would be the injectors ..as you all no doubt know I had a bad one [No4 ] on my car and it only seemed to play up worse when the engine was cold ..not saying tho that it didn't happen when the engine was hot as it did that as well from time to time ..when it happened the engine light would come on and the engine would misfire and become sluggish but if I booted it usually it would clear but the light stayed up on the dash.. three start ups later engine light would go out until the next time it would happen ..expensive job injector £104 and labour £175 the fuel rail is located under the inlet manifold so its quite a strip down to get at it ..last resort would be the head and that could be leaking valves that would be causing the loss of compression you havnt mentioned water loss so I am ruling out a blown head gasket for the diagnoses 
hope that helps
hope that helps 
