Its Accelorometer is a little sensitive.

You can disable the orientation switching outright by going to the Settings.

There's even an app called Auto rotate on/off in the market.
I use it, can't stand my phone switching orientations when I don't want it to.

 

I have the other problem sometimes, can't get it to change orientations no matter what I do...

 

Tried the G-sensor calibration?

 

Now, how do I get to it?

Thanks!
Doc

 

Menu > Settings > Sound & Display > at the VERY bottom of that menu is the G-Sensor calibration. Just lay it on a level surface (as level as you can find, anyway), and calibrate.

 

Place the phone on a flat/level surface.

Menu > Settings > Sound and Display > at the bottom G-Sensor calibration

 

I know I saw it somewhere, I'll give it a shot.

Thanks!
Doc