UK Car Repair Questions? Ask a Mechanic for Answers ASAP
Do a compression test as this engine is a interference engine , meaning valve to piston
will connect and may bend valves .
In your case may be just a slight bend that is causing you the bad idle but at higher revs .
so a compression test should be done and a cylinder leak down test to make sure the valves are closing fully .
Do this then please get back to me .
Good work good find .
Happy motoring .