yea the other card switching to 2d clocks def. sounds like some kinda overtemp protection kicking in, since the 2nd card is hotter. have you tried swapping them? the faulty one as the first card and the good card as the 2nd card? see if the good card in this config does what the faulty one was doing... if so then you have a heat issue. hence why boosting the fan speed helped the other guy. try to add another fan near the cards to draw in cool air for them and see if that helps. 86c is pretty warm, almost 200degF
also what powersupply do you have? an overpowered supply (a weaker one trying to run too much) can actually cause parts to heat up too (from what i've heard). oh i see you have a 950w, i guess that would be enough but you do have 2 high end cards along with a 3rd card running. I dunno what the amps on the rails and such are.
you may also want to run a mem test program for the heck of it.
edit: the temp difference is due to the cooler design and how airflow gets affected when multiple cards are near each other