Twitching boat problem

[Ares1978]

The subject really says what the problem is.
I made this boat, and in the beginning it worked fine.

I tested it using the desert island and sometimes with Everon and like I said, it worked fine.

Then I started making the details and tested just using the desert island. But now that I'm nearly finished making it, I noticed that the boat shakes or twitches whenever other objects come into view. The more objects there are, the more it shakes.

Where did I most likely screw up? It worked fine before.


And I also have a second problem: the rifle reticle disappears when I look at the boat. Well, it more like disappears behind the boat. What can be done about it?