those gpus are easily getting towards 70-80C if under load.
dont remember the temps when r370 starts throttling, doubt it was below 90C,
but try to improve (case) airflow to help cooling (lower/front as intake; higher/rear port as exhaust)...
get the msi afterburner, install without RTTS statistic server..
run it and change settings-->interface to smi big edition v3 (easier to read)
and use it to get an idea of avg/min/max temps/fan speed etc.
use the pc for at least 30min prior gaming so it warms up a bit, play for about 10min, then check the numbers...