99 won't start, now cuts out

I'm a subaru mechanic, but work on everything else as well. My fathers 99 jetta,2.0 is driving me up a wall!!!! I have been driving it on and off for a months now and have not been able to diagnose the problem, cuz it only happens when he is driving it.I explained to him that i can't fix it if theres nothing wrong, but he wants it fixed. Initially, would crank but not start. This lasted for about a month, and only happened rarely. it would start right up once sprayed with ether and run fine. At this point i put in a new ignition switch, cuz the older VWs always had a problem with the ignition switch cutting out the power to the fuel pump relay. that didn't solve the problem. It then spawned into cutting out while driving. when the car is running, it's perfectly fine, no hesitation or sputter what so ever. once again, pulling over and spraying with ether, it would start right back up and run fine. now spraying it does nothing, just crank crank crank. had it towed to my shop where after sitting for a day, it started right up. Put a new fuel pump in and let it idle for about 3 hrs, no problem. drove back to my fathers house to pick him up and it cut out as soon as i turned onto his road. I did not change the filter, because of how well it does run when running, his check engine light is on because of low converter effiency (i cut, hollowed, and rewelded). thought maybe the computer was leaning it way to much, but haven't been able to look into that possibility. the three times it DID die on me while driving, i had let off the throttle, and taken a hard left turn, but personally don't believe that is relevant. going to recheck tomorrow for fuel pressure, spark, and injectors,(assuming it doesn't start) but am not sure what to think of the problem, so some feed back would be appreciated. i'll repost if i can figure it out, incase anyone else has a simular issue

catalyst is covered under extended vw warranty to 120,000 miles on 2.0. hopefully you didnt actually "cut, hollow out, re-weld" the cat that could be replaced under warranty for free because if you did it wouldnt be elegible for warranty.

check cylinder compression i bet its low also.