91 200q pushing H20 out ofcoolant tank

["Beer, Jerry" <JBeer@BooseCasey.com>]

Need the collective wisdom of the list. Title describes problem. With only
moderate boost(14psi) hard driving (sustained boost) causes water to push
out of overflow tank past the cap.  Replaced cap and new cap doesn't seem to
work as well.  I know water pushes back when you let up on throttle, but the
pressure is pushing past the cap.  Aux rad cleaned  and checked 18 months
ago, and main rad replaced with metal unit of exactly the same size. Problem
is only for spirited driving. Daily driving ok, and since I replaced
expansion tank (small crack) no fluid loss.  Suggestions?  TIA
Jerry
91 200q 20v