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It sounds like a bad connection, either in the wiring or in the sender.
The attached link shows the Mercruiser engine wiring.
Common instrument connections are: +12V - I - purple; ground - G - black; backlight - L - blue and signal - S - color-specific to transducer.
It may be that the fuel gauge wiper has crud on the surface. For lots of fun, unscrew and remove the fuel sender. Then do a visual inspection of the wiper, it is probably visible. Clean if required.
Before replacing the fuel sender, turn on and manually move the float while watching the gauge. Note the gauge moves slowly, even though the wiper is moved quickly.
For the trim, my guess is the sender is wired wrong. It looks like a two wire sender, maybe swapping those wires will fix the problem.
https://www.boatdesign.net/attachments/merc-instrument-color-code-jpg.101900/
https://mahorkka.com/i/letusfixitinfo/415/mercruiser-trim-pump-solenoid-wiring-diagram-30-and-tilt-motors-parts-wirin.jpg
