I was away for two weeks so wasn't able to reply. I'll try to record more variables. However just before I left, I received my new plugs and replaced the old ones. The strange thing that I noticed (even though the garage told me they were fine) is that 2 of the old ones where drowning in oil. Any idea of what could cause this? Leaking head gasket? Or is it more likely to be leaking oil springs or valve seals?