Help with set-up

I'm just getting back into vinyl, and have a question. I just bought a new mmf-5 TT. When playing records, the sound quality degrades as I get more into the record. E.G. the first track sounds good, but by the last track on the album, the sound is horrible. It's kind of muffled, and there's some distortion especially with vocals. I'm sure that this is a common set-up error, but I don't know where to start making adjustments. I believe I have the anti-skating weight set-up properly, and I have the cart. weight set to 1.7 gms. which should be correct. Beyond that, I'm lost.

Rob