'93 Thompson 18' Mercuiser Alfa one, 4.3.

When in Forward gear (slow) the back end of the boat is going to the far
left and then to the far right, none stop. Yes, I have a Trim Tab!
I'm always fighting with the steering to keep this boat in a straight
line when going down a river or entering a spring when driving slow.
With full throttle it's fine, does not pull. I can Trim it up, or Trim
it down and you can look behind the boat and see all the zig zag bobbles
from the boat. You can actually see the water pushing from the Propeller
and see the water go from one side to the other; however, the outdrive
it not moving. Weird, huh? There's no slack in the steering. it's all
tight. The Power Steering pump is bad but I don't think that would give
me this problem.
Does anyone have any ideas???!!
Thank you
plz... no e-mail.