Speed change from stylus drag is cumulative over the course of the entire LP side. Also, any bearing drag or any other things could enter into this equation. Basically, after you set the speed at the beginning of play, anything that drags it or holds it back at all, can slow it down during the play of the LP. This may differ in severity depending upon the TT in question. Some TT's may not exhibit this characteristic at all, if certain conditions are present.
I'm not sure why your speed changes only when you touch the platter or bearing structure. That's a new one on me. Maybe you are low on oil.
I'm not sure why your speed changes only when you touch the platter or bearing structure. That's a new one on me. Maybe you are low on oil.