I haven't been a Frost tank in a month or so, but you seem very well geared.
A few questions:
1. What kind of DPS is your raid putting up (2.5k/ea? 5.5k?)?I know it'll mess up you initial HB, but if you're struggling with Group aggro, I think the surest response is to drop a DnD first. It does significant damage and, what's more, it has bonus threat built into its baseline.
2. Are they stealing aggro on single targets, or is it an AoE problem?
If it's single target, look at your Recount or WWS parses. are you getting parried a lot? You might (Sorry...) consider switching to a 2 hander. You have to land hits to generate aggro, and if you are parried for the first 3 seconds of an encounter your warlocks 8k SB crit is going to take aggro, period.
Just throwing spaghetti at the wall here.