2000 V70XC LPT 2.4 195,000 miles.

I'm wondering if this car needs back pressure to function properly. I removed the two mufflers at the back (sounds beastly) and took it for a drive. Sometimes (not all the time) when I floor it, (or more than half-throttle) it'll cut out (acts like it is running lean or misfiring, no CEL flashing) for a moment or two and then continue accelerating. The only thing I did besides cutting the mufflers off is run wiring for the tow hitch and trailer brake controller. I don't have the maintenance history for this car but it appears it was always done at the dealer. Even the battery is a Volvo brand. So, I don't know how old the spark plugs and fuel filter are. It could need a tune-up but it is odd it acts like this right after I cut the mufflers off. I do have a front O2 sensor code (still waiting on the part) but the car had that code when I bought it and it didn't cut out like this.

I'm also experiencing an issue with the headlights. I think the relay is bad. It was intermittent and pushing the relay down would restore the headlights (and blinkers, reverse lights, seat belt warning, rear foglight) and then it stopped completely. It looks like a standard Bosch 5-pin but it doesn't act like one. I had a Bosch 5-pin on hand so I plugged that in and the headlights wouldn't work at all. I took the ACC relay and put it in the headlight spot and that works, though I can't seem to turn them off when the key is on. No matter the switch position, the headlights are on (except in parking only) when the key is on. The Bosch 5-pin went into the ACC spot and I have all interior functions except for power windows. Is there an off-the-shelf option for this relay or am I going to have to order a Volvo relay?

EDIT: I restored normal function to the ACC circuit by swapping the Bosch 5-pin with the starter relay. The starter works normally so it appears my issue is solved. I will be looking to get the Volvo relay to put back in the starter position just for OCD sake.