Engaging APPR

Hello. Often when I engage APPR when flying, my plane will bob up or down erratically. I understand that this is occurs because of my glide slope, but this occurs even when my glide slope is centered. I watched the YouTube tutorial and applied everything, and this still happens. The only thing I am considering is that in the tutorial it says to descend to 3000 AGL about 15 miles away from your destination and then engaging the APPR (which i haven’t been doing because I usually only reach 3000 AGL on final). If I do this, should my issue be fixed? Do you guys have any other suggestions?

What is your aircraft type, speed, flaps setting, trim, and altitude when enabling APPR?

Usually a 737-8, 145KT, 22% Trim, and full flaps. Are there specific settings that I should be flying at for different aircrafts in order to engage APPR?

Try this:
http://flightsim.wpweb.fr/infinite-flight-airbus-a321-200/
It’s a solution to compensate the brutality of the IF A/P.

That’s a bit slow for 15 miles out, which may be causing the bobbing and near-stalling. At 15 miles out, I prefer a speed in between the range of 170-190, and then as you approach 5-8 miles out, configure for landing.