Erratic tachometer reading

[Wheatpower]

Bought boat used in June. Since I've had it I think I'm seeing there may have been some problems in the past. It runs good and floats but little gremlins keep showing up.
Early in my ownership the tachometer was reading high, like pegged out at mid range rpm's. After a short period of running that day it seemed to settle down and read normally. After wiring a depth finder to the battery this past week I took the boat out Saturday. Started great, ran fine but the gremlins returned. I disconnected the battery when I wired the electronics but that is all that I did to what was in the boat. The trim gauge isn't reading correctly either now. This appears that it may be a grounding issue but I'm curious if anyone has had similar experiences or what should be my next course of action?
Thanks in advance for any input.

[Team Stratos]

For the tach, Remove and cycle the rotary on the back a couple times. Be sure to return to correct position. For the trim gauge, check the sending unit on the tilt bracket just below the cowling. There is a tab mounted on there that can become loose. Just return to correct position and tighten. Look a couple threads down at "Trim Cam Sender" post.