Regarding the task manager warning, that's self explanatory to the point of something is maxing out resources. Regarding which app or process, next time you get the error open it and see which app appears to be consuming the most resources. It's going to be an installed app, just gotta figure out which.
The mobile IP error can come from a couple reasons. For example if you don't have a tethering plan, but launch the stock hot spot app you'll get that error. If you're getting it while roaming you may want to ensure call and data roaming are enabled, and warnings disabled. Menu>settings>wireless>mobile network (or something to that tune). Sorry I can't be more specific but I'm currently on my NS4G which has ICS so the menus are different