My wife drives an 86 300SDL with around 165k miles on it. Over the past few weeks, it had been using coolant at the rate of about a pint every 500 - 1000 miles. The low coolant light came on awhile back, so I topped it off with nearly a pint of MB coolant. Since there seems some type of trouble, I have been driving the car. Today, the low coolant came on again (and needed another pint of coolant to top it off).

Strange enough, there are no leaks around the engine compartment and its never left any puddles on the garage floor. There is no excessive smoke and after about 5 or 10 seconds of typical rough idling when cold, it runs fine. The oil I just drained looks to be clean with no coolant traces and the overflow doesn't have any oil spots floating on the top. It does get a little warm when sitting in traffic running air conditioner, but the secondary fan has been fussy. No coolant smells in the cabin either.

Among several other things, we had the head and gasket replaced about 60k miles ago, as there was oil in the coolant. I find it hard to believe the gasket could be gone again.

The radiator is almost brand new, as is the resivoir cap. This is really got me stumped...

Any ideas as to what this could be? If it is the head gasket, how much estimated cost will I be looking at?