2000 Buick Century Misfire after driving for an hour or so

[r22j]

I have a 2000 Century that drives perfectly fine in town and during short drives. Occasionally, I have to take it on two hour drives. During those drives, sometimes (maybe 25-50% of the time), I'll be cruising at 75 mph on the interstate, and the car will begin misfiring and lose most of its power. Sometimes it will throw a P0300, but other times nothing. Once it starts misfiring, I limp it to the next exit, and it can really only maintain 30-40 mph while its misfiring. If I restart the car, it will continue misfiring, but if I disconnect the battery and reattach it, it will finish the rest of the drive just fine like nothing ever happened.

Recently replaced coils, plugs, wires, fuel filter, b1s1 (upstream) o2 sensor.

My only idea is that there may be a sensor that is having issues intermittently. Even after replacement, b1s1 o2 sensor will throw a pending code (P0131 or P0171 if I remember correctly) every couple of months or so when I've checked, but it never actually runs as a full code. Only real code I am running right now is for a wheel speed sensor. Probably wont be relevant and/or helpful information, but who knows.

Thanks in advance for any help!

 

[02LTD2.0]

P0171 is a lean code. You need to check for vacuum leaks.
P0300 is random multiple misfires. Could be caused by a failing Ignition Control Module.
P0171 could also be failing intake manifold seals.