Turntable Problems

My father purchased a turntable recently (VPI HW-19 Mk 3 with Sumiko arm, don't know which one). I have had it in my system until his new shelf is completed to hold the turntable (very big unit). The crtridge is a Grado Reference Platinum. My equipment stand is not big enough for the unit so I put it on a footstool nearby (I know it is a bad placement but I didn't have anything else to use temporarily). It sounds good but whenever there is anything that is really high freqency or very dynamic in the upper range, the unit starts making weird noises that are kind of static-like. It sounds like the stylus is skipping (but that is a guess). It happens on multiple records, some of them being virgin lp's. Does anyone have a guess as to why that is happening? Thanks!