I did a rough estimate based on your pre- and post- boil gravities; anyway, that's way too much. The homebrewers' myth of the vigorous boil has to go. It takes around 30 min on heat (just on heat, not vigorous) to convert the precursor SMM to DMS, and then simply uncovering the kettle will vent off the DMS in a few minutes. All that boiling is just degrading proteins (reducing body and foam) and driving off aromatics while creating caramelized products with much less pleasant flavors than those produced in malting, among other damaging effects. A little circulation to coagulate the break you do want to get rid of, and enough heat to extract your hops, is all that's needed. Technically, it is not really necessary to boil wort at all, though with homebrew equipment we're stuck with it to some degree. Oh, and unless you're using certain extra pale Pilsner malts, DMS is not an issue anyway.