I had horrible back pumps on pmag as well.
I had done two cycles of h-drol previously, and saw a lot of people recommending to dose pmag higher.
My plan was 75/100/100/100/125/125.
I did the first three weeks without any problems whatsoever. Going into that fourth week, I started getting back pumps despite dosing taurine at 5-8g a day. For the fifth week, I decided to stay at 100 and upped my taurine to 10-12g a day. By the end of that fifth week, the back pumps were so bad I was literally crawling to get out of bed in the morning. That saturday, I was ready to quit my cycle there as I was barely moving, walking like an old man bent over with a cane. I ended up taking only two doses of 25mg that day with the first dose at around noon. I went back down to 75mg for the remaining week and it got a lot better.
If I ever run pmag again, I won't go 100mg for more than the final two weeks.
I doubt i'll run it again though. I have half a bottle left, and the couple of places I got the original bottles from are all out.