What do your spark plugs look like? Have you pulled them?

Some will tell you that at 250K, not only are your seals gone, but so are your guides. That's a valve job - alot more than $500.
You'd need to get the seals off to get an idea of how much guide slop there is in the head.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"