I have a 1991 BMW 525i. Little 193hp 2.5l DOHC I6 with a 5-speed manual. 260,000 miles on the original motor/trans. Ever since I've gotten it it's run like poop once it warms up. No check engine light, but within about 5-10 minutes of perfect running, it'll stumble, lose most of what little power it has, etc. Very annoying to try and daily drive. I replaced the bad O2 sensor, which helped but didn't fix it, and then tried unplugging the Coolant Temp Sensor, which caused more problems (Good). Well from the advice of a fellow BMW enthusiast, checked my intake elbow that connects the MAF to the throttle body. Who does this to cars?

The first pic is how much electrical tape was wound around the elbow. I went to the junkyard, and wouldn't you know neither of the 5 series had the elbow still. I'm going to see if I can't find one online, as I think it's beyond repair. Unless someone has some electrical tape they can send me.