You shouldn't use your phone at this frequency for long time use.

- First, the chip is not designed to run at those freq.
- Second, not all N900 are equal in how they can handle this. So some will be able to, other not or using very highvoltage not recommanded if you want to keep your N900 alive for long time

And again, overclock your N900 to thos kindd of freq will make you phone age quickier than normal and you may face system file trouble or problem of that kind.

Now the usual warning have been done, back to your problem, did you disable SmartReflex when you configured freq to 1150 MHz?