TLDR: Check Fuel Injector Harnesses are not broken and loosely connected if you are having "Restricted Performance" and misfires.

This is a cautionary tale about what can happen when your X100 has misfires that don't get fixed right away.

A few months after I bought the car, I took it to a "bad" place that did some needed work on the heater and also some (maybe) needed work on a fuel injector. They also did a bunch of other work that was total BS. This place was obviously a crooked outfit, so I never took my car back there.

After the repairs the car started to have "Restricted Performance" show up on the dash, codes P0303, P0304, P0305, & P0306 (misfires on different piston numbers) and started to leak oil. I took it to another place and they replaced the oil pan gasket but couldn't reproduce the engine misfires that were causing the "Restricted Performance" message. I continued to drive it occasionally seeing the message. I eventually noticed that the engine would display the message and falter at higher RPMs. I started to drive really slow to prevent the message (exactly NOT why you buy a car like this.) The issue seemed to be intermittent. Sometimes revving the engine would help, sometimes not. After checking here and other places I realized the likely problem was that my catalytic converters were disintegrating and blocking the exhaust. This was intermittent based on the orientation of the broken bits of catalytic converter bumping around in the exhaust system.

Of course the problems only got worse and eventually the car stopped driving normally and was basically undrivable. In addition, the oil pan and other gaskets in the crankcase, as well as the rear main seal, all started to leak. This was due to high crankcase pressure resulting from the blocked exhaust. I made plans to replace the catalytic converters and seals but still didn't know the root cause of the issue.

On advise of an excellent mechanic (managed a 10 bay shop and also happened to be my next door neighbor) I realized I needed to fix the root cause or the new catalytic converters would suffer the same fate. He helped me look at the car and noticed a few of the fuel injector harnesses were loosely connected. (Believe it or not he could tell this by listening carefully to the car.) The harnesses could be pulled off with no pressure at all. Plastic bits in the connector had broken off and when they did the mechanic just pushed them back on. I suspect it was the Bad place that did this (Argggg!!!) Because the connection was faulty the fuel injectors were causing incomplete ignition resulting in unspent fuel going down the exhaust and into the cats. This caused them to corrode.

After replacing the and catalytic converters, the car is again back to normal. No misfires. Along the way I had to replace the rear main seal (twice) and oil pan gasket (twice.) So basically because someone decided to not replace a broken ~$16 part, and just pushed it back in place, I spent over a year in a hobbled car, $4-5k in expenses, and tons of my time chasing windmills.

Takeaways: Misfires are serious! DO NOT ignore them! Catalytic converters usually don't go bad unless something is making them do so.