Turntable sound bad

I have a Rega rb250 with mitchell technoweight, vta adjuster, dnenon dl 103 cartridge. A Rega Planar 3 plinth, with aftermarket steel spiked feet. The problem I am having is setting it up to sound right. After the first few songs play, the voices start to sound hissy, and progressively get worse toward the last song.

I am wondering what would cause this: turntable being out of level, Vta, cartridge overhang, Tracking weight? Obviously I am not doing something right, and this is way beyond my limited knowledge. Oh well I hope someone out there can direct me what to try first. Thank in advance, Jason.