Hrmz, well...not really sure why your PH will shoot up that much....did you read the instructions on the back and follow the recommanded dosage? I use the same buffer as you....what i do is measure my KH and then dose accordingly to how much i want to increase KH level. Oh, try using a measuring spoon for more accurate dosing. Can use those meant for cooking, those small plastic ones in a bunch in many different sizes...
Meanwhile, i guess you can do water changes over a few days and see if it'll help. Oh, will recommand you to get a PH monitor....a very nice and convenient piece of equipment to have..