It is unlikely that a I7-4790K upgrade will fix your problem.
Few games can use more than 2-3 threads so the extra I7 hyperthreads are not likely to be any help.
Yes, the I7-4790K will be better binned and can potentially get you a higher overclock.
But you should be able to oc your I7-4670K a bit higher.
Perhaps you can OC higher.
If your vcore is under 1.30v, you can go higher.
It would not alarm me if your temperatures under load were 70c.
How do you know your GTX970 is not at issue?
Is there a possibility of thermal throttling?
What kinds of games do you play?
Update...
Since the problem has only started recently, what has changed?