Upon noticing dimming of lights, I discovered low voltage reading at receptacle and a reading of open ground on my plug tester. I opened panel and checked load voltage on main breaker and also checked both busbars. I discovered when all breakers were shut off I had proper voltage and busbars and also on lugs of the load side of the main breaker feeding panel. When breakers were turned on I measured uneven voltage 112 on a phase and 131 on be phase all the way up to the load side lugs of main breaker including bus bars. All connections at panel look good. Plug voltage measured 76 volts on some and 130 on others. Does anyone know what may be causing this to happen?

Sounds like a bad neutral connection either at the neutral bus bar, or somewhere upstream of the panel. You probably need to call the power company.

This is an open neutral condition, most likely at the utility connections, and needs to be corrected ASAP, as the voltage can rise even more, and do serious damage.