speedometer jumping around/ odometer turning while in park / misfire / ground issue?

[Ryan Forth]

First time posting but have searched the forums for years and have always found what I needed. Well this time I'm stumped, so I have a 1994 gmc k1500 pickup with a rebuilt 350 tbi bored .030 out with a stage 2 cam and long tube headers. (About 4000 miles on engine.) new distributor, plugs, wires, vss, 

 

So here's what's going on, the speedometer jumps around super erratically but when I turn the heater on it seems to change how much the speedo jumps around. If its on high it jumps around the least. Low or off its just bouncing off 85mph like bouncing to the point I can hear it slamming into whatever stops the needle from going any further. When I turn the turn signal on left it makes the radio light dim with each flash. The odometer turns constantly when while sitting in park. I have replaced the vss at the very end of the trans where the rear drive shift meets. I was drive the other day and the truck just didn't want to move seemed like it was just constantly back firing or misfiring. I have been trying to check all the engine grounds but not sure what all needs to be grounded. Any info/advice would be greatly appreciated. 

[Brian Lindsey]

check grounds under dash