Lowering GPU clock causing instability

Okay so a while after I overclocked my gtx660 (not a severe overclocking, I didn't touch the voltage) I realized that the setup I had was not 100% stable, and that the driver would occasionally crash. I decided to try lowering the clock speed of the gpu by just a bit to see if it would keep it 100%. I lowered the gpu clock from 1188MHz to 1183MHz. But after I did I started to get some very strange visual artifacts that would suggest (to me) that the memory was clocked too high, things such as random flashes on screen. I didn't change the memory clock at all, keeping it at a steady 3304MHz. Any ideas as to why lowering my clock speed would actually produce LESS stable results?