Ok @julesh, it looks like you are dealing with three separate issues.
'Accels inconsistent' this indicates the accelerometer calibration is not great. You can fix the error by recalibrating the accelerometers (advised), or by disabling the 'INS' pre-arm checks on the Safety tab of the Vehicle Setup page.
'Lost manual control' this is a failsafe mechanism, and the autopilot is letting you know what it has not received a joystick update for at least one second. If the autopilot is missing the joystick updates very often, it indicates a poor communication link between the ROV and the computer. This can be caused by loose wires, or by QGC having trouble keeping up with all of it's tasks. QGC tends to slow down a little bit when switching between different views, which can cause this to happen on slower computers. You can increase the 1 second timeout from the Safety tab of the Vehicle Setup page (advised), or disable the failsafe altogether.
The video lag could be another symptom of the manual control issue (loose wires or slow computer). Can you describe the lag like how much delay there is, and what kind of frame update rate you see?