Good to know that you solved it.

[FIXED]

The solution was simple. I checked the voltage stability and it was good, so I rewired with a better quality piece of cable and soldered the connectors on the ends and heat shrinked them.

This was the solution. I think the original wire I used was either not connected well enough, was too low a quality or was getting interference.

So if anyone else gets this problem, make sure your wires are good quality and have soldered on connectors.

I'm in the same lineup, but as I understand it, the size of the wire is dependent on the current (amps) it needs to carry, not the voltage. Consider how small conductors get inside the device you are hooking up. A tach would not need large wires to register the voltage on/off from the coil. It might need a slightly larger wire for the on-board illumination (the light inside). A Mini friend many years ago installed a tach the day before heading off 1,000 miles to university. When he started the engine and was sitting admiring how the needle moved, his mother noticed smoke rising from the engine bay. He hadn't fully inserted the bulb holder into the tach - it fell down and shorted, heating up his wiring loom. I was called in to sort out the mess. He was on his way the next morning... with a life-long fobia of electrical wiring.

I'm the last guy that should be giving advise on electrical matters, but I always thought the larger the wire, the better; less resistance, less likelihood of breakage, etc.

Thanks. Is there a specific gauge of wire I should be using? The one out of the back of the gauge is absolutely tiny.  I am using 14 gauge I believe. Could be too much resistance?

Original Smiths tach would behave irregularly if the connections on the coil were not perfect. Try cleaning the connectors and maybe squeeze them together some.

On the other hand, if your Smith's taco looks like this, disregard my last post.

I have the same tach. It would behave itself for a couple of minutes, then the needle would jump all over the place. After checking and rechecking my wiring and the dip switches, I finally gave up and took the car in the an auto electronics specialist. He charged me 100 bucks to tell me my tach was broken. I returned the tach under warranty and got another one that did the same thing.

A guy at Cars & Coffee said all I needed was a tach filter [LINK]. It seems that the newer Smiths Tachs (and many others) are real fussy about the signal they receive from the coil. I installed one of these tach filters and all was well. You're welcome.

I just replaced the condensor, points, wires, arm, cap, leads, coil.  
I doubt it that as the car is running super nice. 

Wonky condensor??

Just purchase the RVC1006 smiths 8k taco. 
Wired it in as per instructions - Red/Black to - on Coil

Car starts up fine, revs up to 3000k then drops like a stone to 0 then picks back up again. 
It I build the revs slowly it does the same thing. 

Its a negative ground, points, 1962 A Series with an MG 1098 engine. 

I have the DipSwitches on the back set to 2 PPR 

On On Off Off Off Off Off On

As per instructions. 
Here is a link to the instructions : 
https://www.7ent.com/pages/installation-instructions-smiths-tachometer.html


Any advice? 

Thanks