In addition to clamping - You may already be doing these but:
1. Purge your injector of air bubbles prior to injecting. For some reason, on some molds this is more of a necessity.
2. Try shooting the plastic cooler closer to 320 versus 350.
I have noticed for myself that when I shoot cooler I have less problems with air pockets, voids, ect. Especially with molds that have bellies (swim shads, toads, ect)