Replaced the rotor cap and a harness in the distributor. The average mileage gauge is now incorrect. Shows 15MPG and jumps back to 0 then goes blank. This was and is before I had the cluster redone ( it was time). Don't know where to look for problem. Could I have done something in the distributor to cause this? 1989 and runs like a bear all stock. Do I need to have a shop look at this or can I repair it?

TIA
Ken