Jan,Do a compression test mate this could be possible head gasket blown????or get a sniff test done at your local garage to find out if you have exhaust gases in the header tank..............

Just a thought because you have changed all items that are electrical,maybe time to start looking at the engine itself???

On my stg 3 cossie my plug gaps were 0.6mm on escos pp8 plugs

So what i would do is set plugs at 0.6mm, and then a compression test.

They can be a pain i know i,ve 2 BUT keep with it

Paul