Room temp. can most definitely affect PC temps. And a hot PC blowing hot air out the back all day can even make a room's temp. rise, over time.

I use an air conditioner in my room during the summer for precisely that reason. However, the weather temps you stated shouldn't cause the pc temps to be that high, unless your room itself is actually getting much hotter, or you have very bad airflow in the case. Maybe 50C, but 70C is getting up there for idle at those room temps, imo.
Make sure all the fans in your case are actually working, not just the one on the GPU. And if your pc case has a front grill with the foam lining, take that off and bang the dust out of the foam, that can block airflow a little if it builds up too much, too. If cleaning/dusting everything doesn't work...it may be time for a new card, imo, yes. Could be it's been running too hot for a while now, before you noticed, and something is finally giving out.