When was the Cambelt last replaced? It's best practice to change a cambelt every 100,000km for that very reason.

If it was loose (enough) it could have jumped a few teeth, then jammed.

kimber7wsm wrote: On our car, a tiny piece of timing chain guide (which is made of plastic) broke off when I hit a severe bump. This fell down and ending up going between the crank pulley and the timing chain, causing it to jump a couple of teeth. We were really lucky in that while it buggered up the timing, it didn't jump far enough to cause damage. Took quite a while to work out why the timing was suddenly out, as it affected the low rpm a lot  more than high RPM. Maybe something like that happened, or as Alan said the tensioner could be not working. Was the tensioner replaced with the cambelt?

Tensioners . Yes I know from talking to a couple of mechanis when changing belts they sometimes do not change tensioner. They carry out a bench test for tensioning. My boy did a belt change via youtube. While you have things a part change tensioner/idle pulley,belt,water pump. Toyota camry pre 1990.2.5lt. Parts from aussie all up $180. Had the belt 1 tooth out so lacked power,adjusted it and ran like a dream. Sold it to his friend,done 10 thousand now no issues,2 yrs old.

Quick update in case anyone was interested.  I sourced a set of secondhand pushrods and bought a new cambelt, which the garage fitted before Christmas.  After fitting these it ran fine.  I've done about 500km over the holidays and so far, so good.  I would like to upgrade but suspect that without a lot of luck it can't be done on the budget I wanted to do it on.  Loved the look of the Toureg, but at the age/mileage I could afford trouble with the air suspension seems quite common and very expensive to fix -more than the car would be worth expensive.