A B18 valve problem is beating me up that I'm not sure how to address -- but once upon a time I'm sure everyone was hit with it. How do you clean a sticky head valve?

My long-dormant B18 in the 67 runs great when warm, but until the head heats, it may go through cycles where one of the valves on the #1 cylinder is snail-slow to close. I'm talking 3-5 seconds if the engine is stopped with the rocker in the "up" position. It's caused a bit of trouble in that twice the pushrod has come unseated and then trapped in an unfortunate position when the valve decides to behave. Of course the result is a bent pushrod. Ka-clunk.

Oil is freshly changed with a bit of Sea Foam added at the prodding of a friend. What's the best advice for this valve? I'd like to avoid pulling the head if I could but in a pinch I'm not totally opposed. Would I have to replace the valve guide and all that, or am I staring at a weak valve spring being the issue? Is this something a chemical cleaning agent might be able to free up or am I bound to just bite the bullet and redo the valvetrain?

Thanks all!
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88 740ti 260k and counting....