Exhaust / combustion gasses leaking into the coolant?

Doug

>Hey now,
>I am the proud new owner of a 89 Westy, Rosebud as we have named her. =
>anyway we are having confusing cooling issues. 1st of all the person =
>that sold her to me thought that a head gasket was bad (so we got a good =
>deal) rebuilt engine 20,000mi-she is not leaking any fluid anywhere! so =
>when you drive her on the freeway or up a mtn.aty a good rate of speed =
>the refill tank(behind the license plate) starts to fill to the point of =
>overflow!-this has only happened once as I keep a really good eye on the =
>gauge-as it climbs I let off the gas and get where I can pull =
>over-making sure I don't overheat., really only seems to happen under =
>load and not all the time- so I took her to trusted mechs, thjey drove =
>her for 2 days- no troubles - so far we have driven her 1200 miles =
>-overflowed twice- not quite ready to take heads off yet-sometimes =
>coolant level remains same level overnite sometimes refill tank goes =
>into overflow tank after she cools all the way down-once again no =
>consistency  with issues- going on family vacation in 2 weeks so trying =
>to get bugs ironed out..please help
>Michael