As far as that part goes you may have a real hard time finding it, one of the other guys on the forum has been looking for a plastic part for over a year now and still no luck finding it. The problem is the system is after market. But you may get lucky and find one in a wrecked 90.
The fuel gauge on all of them does that, and yes it's from the fuel sloshing around, I see about a 1/2 tank swing in mine.
Edit: Oh yeah, welcome to the Forum!
Don't forget to
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first then ask questions later! The loose nut behind the wheel