Assistance Needed Please!

[Jenobaga]

Last year, I inherited my Pops' 2010 Volvo XC90 3.2...

Okay... last night on my way to work, I saw the wonderful red triangle of doom pop up on my dash with a message saying "Power System Service Urgent". Seeing that I was right down the street from the office, I decided to just make it there. That's when my car started making a very loud high-pitched squeal for about a minute. I make it to work and go about my day somehow forgetting all about this situation. Eight hours later, I'm confronted with the same issue minus the noise on my way home. When I got home, I decided to plug in my OBD-II since the CEL was also on.

Here are all the codes it threw at me:

P0303 - Cylinder 3 Misfire Detected
P0305 - Cylinder 5 Misfire Detected
P0403 - malfunction in the engine's exhaust gas recirculation (EGR) circuit
C0700 - malfunction with the exhaust gas oxygen (EGO) sensor
P0503 - The powertrain control module (PCM) has detected an intermittent, erratic, or excessive voltage input

Also... I've gotten the "Slow down or shift up" message several times this week as well.

Any suggestions on even where to start here would be extremely appreciated. Yes, I am kind of a newb, but... I did manage to replace the radiator about half a yeat ago on my own, so I know I'm capable... just more or less lacking direction with all these issues at once.

Thank you!

Jennifer