420SEL High tickover when warm

[Oliverp]

Just one more question!

When cold, the car idles in park or neutral at about 750-850rpm, and engagement into drive or reverse is smooth.

When the car reaches operating temperature, it idles in park or neutral at 1500rpm, and engagement into drive or reverse creates a big 'clunk', and once drive or reverse is engaged, it drops back down to about 750-850rpm.

I'm getting the valve seals, guides and the head done over the next month or so. Will this solve the problem, or is it an idling problem?

Thanks again everyone