Portafilter loosens during brewing, rotates to left and leaks!

Have a problem that is occurring with increasing frequency. After I attach the portafilter to the brewhead and push the brew button (after the green light comes on) the filter and handle rotate slowly to the left and eventually the filter loosens enough that the seal to the brewhead is lost and coffee, grounds, etc. start to leak out the top of the portafilter. I am attaching the filter very firmly but that does not seem to help. I have cleaned the screens and gasket, etc....all looks to be in good order, the gasket is complete and not hardened. Have not been able to find any advice on this particular issue anywhere online. Any ideas/help would be greatly appreciated! Thanks.