Classic signs of Limp Mode, where the ECU regulates engine output to safe levels in the case where the engine cannot properly control charge (turbo) pressure.
Sometimes it's as simple as one of the intake pipes having a loose clamp, but as always it can be a deeper issue than that. Do some searches on "Limp Mode" and you will find plenty of information to help you get started on a diagnosis.
'97 Jetta TDI - 135K - R.I.P. 10/22/03
'03 Jetta TDI - Silver / Black Leather, 161K miles - Sold.
'09 Jetta TDI
Sedan - Sold!
'14 Passat TDI
SEL - Blue / Black Leather
VCDS Available in Chicagoland - email@example.com