I have a 98 mk3 Jetta GL, 2.0 i4. When driving it starts and idles fine, and when I press the gas it accelerates normally for about 3 sends then dies. Dies like the fuel shut off - no electrical dimming or anything like that. If I keep my foot on the gas it doesn't recover, but if I take my foot off the gas and press it again, I get another 3 seconds before it dies again. This happened while driving home and I was able to make it several miles back just by pressing the gas 2-3 seconds at a time, on then off, and it drove almost normally, able to maintain full speeds etc.
Error codes only indicate bad O2 sensor, this has been an issue for months. The cat crumbled and blew out last year, a shop welded in a new cat but couldn't remove the seized front O2 sensor. I suspect the welding fried the components and the O2 sensor failed, but after completely disconnecting the O2 sensor it ran just fine, presumably reverting to factory defaults while the O2 was blind.
The current issue began not long after sitting for months not driving (Covid shutdown I didn't drive anywhere). The battery had gotten fully dead, and after recharging, it ran fine for most of a few days before this arose. possibly the dead battery lost the saved fuel map? Seems to possibly line up with the battery not charging also -
I assumed it might be battery or alternator, but runs exactly the same with a good full battery from my other car, and alternator appears to put out full voltage...