25mm replacement for Bosch/OE:
Haven't tried one of these yet because we haven't experienced the issue you're having, but we use a lot of Turbosmart products and haven't been disappointed yet:
Turbosmart Kompact Plumb Back Valve (Diverter Valve)
Here's the updated version of one we used a long time back, when we were replacing someone's failed aftermarket one (not naming names in case still sold by the host) with another:
Turbo XS RACING BYPASS VALVE TYPE 25
Forge was also a quality OE BOV replacement, but we haven't used one in 15+ years.