H'mmm , whilst Accellera's aren't the best grippy tyres to choose , I have them on the rear of mine / Sessanta's front , running 41psi front 38psi rear (2 or 3 up if towing or consistantly long distance FAST driving)
I admit they aren't the best but am puzzled at oversteer even in the wet at the pressures you say you're running . It's not happened to me in the wet and I don't drive slowly , often dropping into 2nd for roundabout fun but of course they are 235x40x19's which I've heard can make a difference ie. tyre size/Accellera's
I am wondering though if your pressure is correct ie. a garage forecourt or your own accurate gauge . I have a twin cylinder Michelin foot pump plus race gauge in the boot , as I compete in my other car , so can say that accuracy is spot on. Try dropping the rears 1.5 to 2 psi --- have a good read of the handbook for your specific model/tyre wheel size.