PT crashes with audio suite all the time...why?

I have 6 X 5.1 stems 90 minutes each on my session (master, M&E, DX, SFX, Music, LtRt). I need to drop the volume of all stems equally by 1.5 db, I select all stems use the digi gain plugin and dial -1.5 db an click render. PT gets stuck on beach ball every single time, I have to re-start PT... why! So instead of using the PT gain plugin, I try the CEDAR adaptive limiter (threshold set at 0dB, so there is no limiting, ceiling output set -1.5 db). PT gets beach ball every time, it makes no difference. Why is it that PT can not handle 6 X 5.1 stems, 90 minute each at one time for gain processing? I have to process each stem separately one at the time, which means I can not batch process all stems at once overnight. This is supposed to be 64bit and this kind of problems should not exists.