Welcome !

If you think the temperature is too high, just reduce the frequency. Even if your hashrate will not be the best, you will not burn your GPU 
Me, I just make my possible to stay arount 70-75°C, not more. I don't know if it's the best choice but in my opinion, it seems fine.

350kh/s with an underclocked (at 610Mhz) 7970 to reduce heat and noise (I sleep near to my computer  )

Welcome 

Hello,

I have the same problem, this is the first time I mine on a P2Pool, shares are extremely slow to send (even if I know it's slower than a classic pool, here it's extreeeemely slow, 10 shares per hour), and my chart looks like yours... But i didn't solve it yet, I don't understand what's the problem. I have a i7 3930k and a hd7970 (I mine with).
I tried to mine with cgminer CLI and guiminer scrypt, guiminer seems to work faster but the problem is still here.

I hope someone will help us, because I don't see what's wrong !