Hi John,
Very first thought when I read your post...
Sticking EGR valve.
I had an Avensis with low performance, smoking and shunting about two months ago, cleaned out the EGR valve, problem sorted. Met the owner again on Monday for a problem with a different vehicle and it's still going fine apparently. The thing we noticed at the time was that with the MAF disconnected the vehicle performed much better. Presumably with a MAF code logged the system stopped applying any EGR. On this basis the customer asked me to fit a new MAF. It was after this I found the EGR fault so I now have a known good MAF for testing purposes .