I've been priming by dissolving corn sugar (boiled with dechlorinated water then cooled with an ice bath), then gently mixing in the primary before bottling. The first time no problem. The second time I've had some bottles that were gushers and some that were a little on the flat side. Third time I cold crashed and had a few that were OK, some totally flat.
I'm afraid to mix too vigorously so as not to disturb the yeast and also not to let much oxygen in. Any tips on consistency?