Yesterday I'm in 5th gear and gave it gas and car ran SMOOTH, but noticed that the tach was jumping, only cel is p0420-has had this for months & Im sure its not the issue. I then noticed that it did this in almost any gear and did this around 4k rpm. Any ideas on this please.

 

Guessing after market tach? Check your wiring.

 

no, factory...all wires seem ok

 

I would fix the Check Engine light first...
P0420 - Catalyst System Efficiency Below Threshold
could be an exhaust leak..damaged O2 sensor...plugged catalytic converter...