The only problems I've had at long focal lengths has been when I'm zooming to 200 and the car is getting even farther away. But when it stays in range, I've never had any problem, at least from the lens' standpoint.
This was taken at 200mm and the Jalpa is perfectly sharp (at least in the original photo). But the sweet spot is around 100mm. Yes I have it on the D3100 which honestly is one of the...I don't wanna say worst cameras, but it's one of the most basic.