Tuning Thruster Control with PID

I have a script that works, well at higher speeds - it responds in a decent time and adjusts thrust accordingly with maybe a little yo-yo effect. But at low speeds its really not very good, if I set my target speed to 1m/s it will do fine if moving flat, but as soon as gravity is involved I get crazy yo-yo'n with the ship moving backwards and forwards.

I am looking for tips on how to tune this to get a good result with gravity. At the moment it will do, but I am a perfectionist, I want my...

Tuning Thruster Control with PID