1998 Sunsport 280 ss with twin 5.7. The port tach doesn't work until I hit around 1000 rpm's then it would work pretty accurate along with starboard side gauge. Problem is now starboard side is starting to flutter around a bit. Especially towards the high end. What would be a contributing factor that would cause both? Could both gauges be going or probably a loose ground wire?
Here is a short video
http://i173.photobucket.com/albums/w78/neohiofan/90B90A04-898F-4041-88CC-AE23D015F398-13369-00000326672F86A4.mp4

I would check all the wires and make sure it's grounded properly, if it's still not working you might need a new one, it's like any other electrical device, they don't last forever, when mine started acting silly I bought two new one's from Livorse and said the heck with it, good luck.

I have twins and just wondering what are chances of both going bad at same time?

Here's the other thing you can do, I've sent one of mine in and they'll check it for you for like $10 or almost nothing and if it's repairable they can fix it for lots cheaper than buying a new one, I did it once a few boats ago, worked great, the only thing is you won't be able to use your boat or if you do it won't have one of it's tachs, I had twins to and I just sent mine in and taped up the wires until it was shipped back to me, call them they can help you trouble shoot it.

Who did you send it in to?

gaffrig or livorsi will do it