Yep, 670's start to down clock at 70c. That is way too high for temps. What case do you have? Are your cards getting enough air? I have a fairly well flowing case, Corsair Carbide 300R, and with two fans feeding a single EVGA 670 FTW. My card struggles to hit 60c during benchmarking/extreme gameplay.
I have a thermaltake chaser mk-1 http://www.flickr.com/photos/84412941@N08/
I have fans in every possible spot that a fan can go my cable management is very good so the airflow should be good
the idle temp for the cards is about 36.
when i play BF3 with gpu fans at full the temps are 69-72