[Ian B Walker]

Dont forget that metal expands when hot. When they expand the valves can remain open a little on the compression stroke. It may give you a slight missfire and defo reduction in power while ticking over. Checking for system air leaks, hot soapy water brushed round the joints will create bubles and show you where the system is leaking. Its just a thought, would love to hear that you get your problem resolved asap as such a car must be a joy to drive.