I have the same problems, and I've tried the exact attempts at a solution that you have tried. Sometimes a game starts briefly, but the rebuffering is constant and makes the game unwatchable. Usually, however, I don't even get that far, and the "retrieving" bar simply grinds to a halt.
No problems watching games on my Mac laptop.
Very frustrating, because one of the big reasons I bought Roku in the first place was so I wouldn't have to carry my laptop, power cord and adaptors, etc., down to the TV room to hook it up every time I want to watch a game on the big screen. As it stands now, my Roku investment is a waste.