Get a boost gauge. “how it feels” isn’t a very good troubleshooting step with software tuning.
Also a vag com would help a lot here. If the battery is good enough to fire it, but not enough to supply power to the ECU WHILE cranking (voltage drops a lot while cranking) it could be resetting as you crank it over.
What I would do is key on, go into vagcom and check the status of the readyness. if all the ones that dont need the car running say pass then you know the last time your drove it they passed and ran fine. Then I would crank the car start it and refresh the readyness screen. if some of the ones that used to say pass, now say incomplete the ECU reset as you cranked it.
Moral of the story, get a battery anyways. Chances are VERY good this is the problem, and chances are just as good your battery needs to be replaced due to age/climate anyway.
Dont have vagcom. Battery tested as good and it only seems to happen when I leave it overnight. But not every time. I will absolutely get a boost gauge though. Any suggestions on make?