Did you also increase things like cpu voltages, etc?
You need to supply the CPU more power if you want to overclock it. It could also be that it's unstable at that speed (800 Mhz is quite a jump). I'd recommend you try increments of 100 Mhz until you reach your upper limit. Remembering to increase the voltages a little bit every time too.
There's plenty of guides on the internet if you're not sure what settings you should be changing, etc. I'm a bit of a novice myself but the more I dabble in it the better I'm getting at it.
Practice makes perfect and all that jazz, lol.