Electrical drain?

To start, my wife blew the engine in this car originally and had a used engine with about 30,000 miles put in (currently about 40,000 now) in the beginning every thing worked perfect until about 6 months later the catalytic converter messed up and is now creating alot of what I think is "back pressure" and is now idoling lower and slower take off before warm up. Since around that same time my power in the entire car (headlights, radio, interior lights, etc.) Seems to be horrible when it idols low but presist even when driving but not as bad. For referrence, my radio gets choppy like loss of power or watts at volume level 25 during idol or braking, but when driving at hwy speed my radio will let me turn it up to about volume level 35 before getting choppy and cutting out (like the more watts needed to play the radio the worse it does, and during braking lights go kind of dim and brighten when I press the gas). Also vehicle is worse when sitting in sun on a hot day? Odd? But I cannot for the life of me pin point it! Would like to fix myself before taking into shop if possible I am very handy with cars and wiring just not understanding problem fully enough to pinpoint cause. Alt and batt have been tested and are fine and of course all fuses in place. ANY INFO WOULD BE GREATLY APPRECIATED! Thank you in advance