Inconcistent Thrust and Sudden Altitude Loss

I have been trying to tune my octocopter, but have had little success. When I enter autotune, my drone suddenly loses altitude, despite being in AltHold mode. I have also flown it in PosHold and Loiter, and when I try going left or right, I also experience sudden altitude loss. I have attached the .bin file, and am not quite sure what to do. Sometimes the drone will hover really well, other times I will be all the way up on the throttle and it won’t even get off the ground. Also, my octocopter tends to wobble in Loiter, but it does not wobble at all in Stabilize of Alt Hold mode (when it’s not erratically losing altitude. Any help would be greatly appreciated!

It sounds to me like it’s under powered.
Could you tell us about your motors, props, and weight?

Unfortunately, it’s on the overpowered size. Thrust is around 40% with 18 inch props.

You have your MOT_SPIN_MAX set to 0.44
MOT_SPIN_MAX: Motor Spin maximum

Note: This parameter is for advanced users
Point at which the thrust saturates expressed as a number from 0 to 1 in the entire output range

0.9:Low, 0.95:Default, 1.0:High