Rough idle/ volt drop

[Jscott]

Hello friends. Another one for you. My 1990 cutlass 3.1. Has a rough idle when it warms up. Replaced a lot of items and even a charcoal canister. Thinking the evap is bad. Which it was anyway. But the car starts acting up when it’s in gear and idling with foot on brake. Which I also noticed as my foot is on brake the engine kinda surges and I see my volt gage dip up and down a little. But enough to make the engine not perform as it should. I have to mention that I have a power master 3 brake system. Not a fan. Any ideas would be appreciated.