Tach problem & Car studdering

Okay i got 2 issues with my car right now…

1st Problem:

My tachometer only works if moisture is under the car. For example, everytime the bottom of the car gets sprayed in a car wash, the tach starts to cling to life and works perfectly for about 20-25 minutes then dies out. If I go through a large puddle it’ll also cling to life but will only last for about a minute or two. If I were to restart the car as the tach’s working, it won’t work again. On rare occasions when the car started in heavy rains, the tach will work upon start up but die out once the rain stops and the roads dry up.

2nd Problem:

Over the past couple of months I started to notice that as I downshift from 4th-3rd or 3rd-2nd my car starts to “studder” (shake violently right after the shift) and the lower i go in gear, the more violent it shakes. Its been getting worse over the past 3 or 4 months. Some say it’s the clutch going but I’m not sure. It feels like it’s coming from the rear subframe like the diff (open…has never been changed) but i was told that shouldn’t be the issue.

Any advice would be a great help as I want to figure out the issue A.S.A.P.
No Internet Gangsters!
Thanks
Rich

tach sound like a bad ground

or a broken signal wire from the speed sensor?
look over the wires.