Why does changing the PCI-E clock stabilize my system?

A couple months ago, I replaced my Radeon 6850 with a R9 290. With the R9 290 installed, my system would very frequently just hang and require a hard reset (usually within 1 hour of use). Nothing was overclocked, and I have been using the same system with the 6850 for years with no problems.

So, I started messing around in the BIOS, and I found that when I changed the PCI-E clock rate to 150 MHZ, the system ran perfectly. I have been running it this way for about a month now with no problems. Anyone have any idea what's going on?

I just tried to read up on it, and all I found were some forum posts. I guess some extreme overclocks can be stabilized by changing the PCI-E clock. Why is this? Some posts also stated that you shouldn't really go above 110 MHZ, and that anything above 130 MHZ is "dangerous." What do they mean by "dangerous?" I don't really know anything about overclocking, but I thought only changing voltages could harm components?

It could corrupt data on the hard drive.

I'm not too familiar with the newer boards, but in the old days of overclocking, often it was the pci bus that would become unstable beyond a certain mhz and as a result the hdd would become corrupted and you would have to slow the system a bit, reformat, and start over.