Many cars have two springs on each valve, a heavy one and a lighter one inside of that.
I cannot explain the engineering aspects, I just assume it takes both springs to cover the wide range of frequency, 500 to 6000 RPM, of valve opening and closing operation.
I could see how one spring might begin to allow the valve to float, say at 4000 RPM, before the second, lighter(??) spring comes into "play".
Many years ago I would have suggested a valve job, but at today's labor rates and with 200,000 miles....
Do you think a compression check would reveal if you are right ?