Vulcan A/P kind of wonky, or is it me?

[Daisrael]

What’s the quick way to just get the Auto Pilot to work? Just got this amazing plane a couple days ago. I’ve read the manual and going through it again. That said I don’t know if the following is just me being a rookie with the Vulcan or what?
Whenever I engage the A/P the plane climbs and dives +/-4000 FPM alternatively. Sometimes it even flies away from the selected heading. It seems like the manual instructions don’t actually apply to the aircraft when in use? Tried while straight and level pointed at heading hold bug, then engaging A/P still it porpoises or turns.

Can somebody kindly reply with a known step by step procedure for holding a heading or GPS and altitude? Cause following the manual isn’t working.

Using FSX SE

Thanks