we're not in a croftside town, but in a 36 Pilgrim. Our master bedroom is above the garage and porch. We are having the same problems as well, though I can see how yours would be more intense with such a large window. (we have 2 smaller ones).
Our neighbors on either side were having problems with their a/c and heat, and both found this summer when someone from an a/c company looked into it, that one of their main vents from the basement was 3/4 closed! Therefore the hot/cold air was not travelling all the way upstairs...as is the problem we were having all summer...our main floor was very cold, top floor was hot.
Maybe look into checking this vent, apparently there is a valve-type thing in it that might help the situation a little. You said you had Mattamy there, so maybe they checked it, but you mentioned insulation and the attic, etc. Perhaps the problem is in the basement.
Jessica
