Can someone tell me how you determine if your speedo diaphragms are bad? Do you even need them?

I have an interesting issue that started to bug me this weekend. Since I have owned the boat (roughly 400 hrs of use as 2nd owner) the analog speedo will "bounce" about 1mph anytime I am cruising between 18mph and 28mph. It seems to level out above and below those speeds. I never really cared...

I have a 2007 X-2 and I need the rocker switch for the Speedo and Display/Selector. Dealer can't seem to know what the part number is. Anyone have any idea as mine is now sticking and I can't get it to adjust my speedo.

My speedo went out and my depth reading seems fishy, I am assuming that my triducer needs to be replaced. MC part number 509135 is $655.23 which is the whole kit to add one. I looked up several sites that say the Airmar DTS800 triducer $200 is what I need however due to proprietary software reasons...

I've got a speedo which was replaced by a PerfectPass system on my 2003 X-10. It was working perfectly when removed, pics attached.

GLS has them new out of the box for $85 I believe, just looking to get $50 + shipping....