I've had the same problem with some frames.
1. Use 16g mains and crosses
2. Use the stiffest poly cross that still feels good
3. Pre-stretch the gut
I'm using 4G and it's very low powered and feels very comfortable in an open pattern.
Pure Storm GT: 337g, 8.5 HL, SW 330
VS Touch Black/Focus Hex 1.23 @ 58/54