What firmware version are you running on the Solo it will tell you in the Solo app it almost sounds like a parameter problem? Also make sure the level and compass calibrations are all correct.
If you want to use mission planner you need to be very careful you could actually make things worse. Do a search on how to use it for the Solo and don't just randomly click settings while you are connected to it..
Oh and when it prompts you to use the wizard to setup click not now...You also want to make sure you are using mission planner in expert mode the first thing you should do is reset the Solo's Parameters Look for " SYSID_SW_MREV" and set it to 0, and Write Parameters , then reboot the Solo. and redo the calibrations on Solo after you do this.
Mission planner is here use the 1.3.56 MSI or zip for windows >>
ArduPilot firmware : /Tools/MissionPlanner