Tach Fluctuates

I have just bought a 93 del sol 1.5 bone stock and since I have had it the tach fluctuates, some at idle but mostly going down the road. It doesn't effect the idle its just irritating. All the plug ins at the back of the cluster are in, I had to remove the cluster to replace some bulbs that were burnt. About all thats been done so far is just a cap, rotor and plugs. Any ideas as to what is causing this would be great!