Sitting at a VIP right now and they have no ideas right off hoping for some quick tips.

1998 Jetta TDI accelerates fine, but when it hits 40mph it starts to lose acceleration, and it jerks back and forth slightly as it decelerates. If I stop the car and wait a little bit, then I can accelerate normally again, but when it hits forty then it will decelerate again. 170k miles. I was told I needed a new timing belt when I bought it 5 mos ago. Is there a connection b/w this and the timing belt? Is it a fuel injection issue?