Was just going to suggest you're in limp mode, but a quick search shows that you asked a question about limp mode in 2016, so I'm assuming that's not the issue?
https://www.taxi-driver.co.uk/phpBB2/vi ... =9&t=30409
Anyway, I'd written this below before I found the old thread, so here it is - it was the battery disconnection and running better when restarted that made me think of limp mode, but who knows
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I'm not a mechanic, and know even less about TXs than standard saloon cars
But at a rough guess I'd say your cab is in 'limp mode' because of some fault or other, so the power is limited and it'll drive like a dog.
You would really need to get it on a diagnostic machine and that should produce fault codes and tell you what the root of the problem is.
But that's maybe why it ran better when you changed the battery - disconnecting the battery would clear the fault codes, so it wouldn't be in limp mode, but when the underlying fault reappeared it would end up back in limp mode.
That might also explain why it's also run better after restarting the car - stopping the car and restarting it might also take it out of limp mode, but it'll go back into limp mode again when the underlying fault reappears.
When my 2006 Passat had done about 170,000 miles, it drove fine 99% of the time, but if you floored the pedal and accelerated really harshly for a few seconds it would go into limp mode, and thus be difficult to drive, but not impossible.
But after I'd stopped the car and restarted it, it was fine again, until I floored the pedal again, and it would just go back into limp mode. (I don't think simply stopping the engine and restarting it would do - it had to be properly shut down and maybe locked up for a few minutes - but it all depends what's causing the fault and how your car is configured for this kind of thing.)
(Never really found out what the problem was - mentioned it to my garage, but because it was easily avoidable I didn't want to spend money diagnosing and fixing it. But I eventually sold it to the garage, and they later said it developed a problem with the turbo, so that might have been the cause.)
Another interesting one was during the Beast from the East, when my windscreen washer bottle froze up - obviously not enough screenwash in it
But when it all thawed out it still wouldn't work, so assumed motor had burnt out trying to use it when frozen. But turns out that if you try to use it and it's frozen, the motor will be disabled, and a fault code will be generated. So disconnecting the battery was all that was required - that cleared the fault code, and the pump would work again
So the pump was in a sort of 'limp mode', but disconnecting the battery simply cleared it, and because it wasn't caused by an actual fault, it's been fine since. And easier than paying VW mabye £100+ just to plug it in to clear the fault code
Anyway, that's my best guess, but as far as the underlying fault is concerned, haven't a clue
But I suspect that if you simply disconnect the -ve battery terminal and reconnect it after a couple of minutes then that might make it drive better again, for a wee while at least. But no doubt it will just end up in limp mode again when the underlying fault reappears.
