@Mark420
Most crashes are caused by heat or power issues.
I don't know how much you know about electronics, so I will generalize a bit.
Power issues. The guts of the chips in your computer are made up of millions of transistors. Transistors are little switches that either allow electricity to pass or stop it depending on a signal they get. When power fluctuates, then signal to the transistor fluctuates and a switch that should be open closes and vice versa. In simple terms it's like someone turns on and off all the lights randomly, the computer goes haywire, and it crashes. Most people attribute power issues to the PSU (and rightly so), but all over your computer are capacitors. These are like little batteries that keep power levels steady. They have a lifespan and as they die any fluctuation in power can cause a crash.
Heat issues. As your chips become hotter, there is a higher chance that a transistor will leak (let current pass when it shouldn't). Too much leakage and you have the same problem as above (all switch on and off at random ... crash). Over time, heat also degrades the pathways inside your chips. As the pathways degrade, you increase resistance. As resistance increases you have more heat production. I think you can see the death spiral here.
So, your 390x ... you have everything going on. It's a older card, so things have degraded a bit. It's a hot running card, so things have degraded a bit more. It's a power hungry card, which means more heat and more degradation. This is why it will only run when underclocked. And now it's at a point that even if something small goes wrong (something it could have recovered from before), you get catastrophic cascade of failures and .... crash.