Sorry if your already aware of this BUT, if you hit "shift+F1" it will toggle you between 6Dof and 2Dof. It's real command is to bring some of the offset targets, like the 109's and Mossie's etc to the centre for you to use. (it may also be for other things i don't know about)
So if by chance you have hit shift+f1 sometime you could be flying around in 2DOF without realising it and that will take away the ability to move in and out plus some other moves that come with the 6DOF.