Pro-ject primary distorts gradually

Hi I have a question that I can´t seem to find an answer for.
I bought a Pro-ject primary for my sons 11th birthday. I hooked it up with a used NAD amplifier and a set of Dali speakers. When we first started playing it sounded really good, but gradually a distortion started creeping in especially in the high notes. I tried giving the system a break for a couple of hours, when I played again the same thing happened. Started of sounding fine, but gradually became more distorted, like almost breaking up some of the music. Can anyone help me with what this can be?
What Cartridge? Which Pro-Ject? Are you using the NAD's phono inputs? If so, does it handle MM only? or MC cartridges? need some more info. Are the records dusty? is the needle kept clean? How old is this NAD?

Matt M
More distortion towards the end of the record?  Check the cart alignment.