Higher temps can hurt the stability, but I would think 80C, while quite high, wouldn't be high enough to affect the stability greatly. Keep in mind, every card is different and will behave different during overclocking. It could be just that your card isn't one of the best overclockers. However, even the most horrible cards I would think would get more offset than you were able to get.
Another thing to keep in mind, is that your power supply is most likely getting near its capacity. Depending on your system draw, I could easily think you are pushing close to 500W which is over 80% of your rated usage. This is getting outside the peak efficiency band of the power supply, which is typically in the middle of its rating, say between 40-70%, of course it varies between brands and quality of power supply. Also, if you power supply is older, they tend to degrade and lose efficiency with age. Unstable or inconsistent voltage supplied to your GPU would be more of a concern regarding stability.
The purpose of applying the additional +38mV is to help with the ability to overclock and remain stable. As one pushes the clocks higher and higher, it requires more and more power to remain stable. This additional overvolt feeds this need for more power. With a custom BIOS, such as with Classified cards and the like, more voltage can be set, but with the standard BIOS we can only apply +38mV, which usually equates to 1.2V as that is the max voltage limit set by Nvidia.
I have had crashes too without getting the notification, so I don't know if it was there but it disappeared before I could see it, or it was due to other reasons. I highly suspect that 99% of the crashes we experience when we have overclocked cards are due to the driver crashing.
Lastly, regarding the memory clock reading, is the 1502Mhz reading at idle or during load? I assume you are seeing the value in Precision X? If that value is at idle, check what it reads during load. It should jump up to 3000 for our 780s as standard.
Hope I answered all your questions, but feel free to shoot me a PM or here and would be happy to help out more.