Unlike Unholy, Frost AOE has no ramp-up time on their threat curve. Howling dumps damage (and so often crits) immediately, which brings a ton of threat with it whereas Unholy takes a few GCDs to really get rolling. Currently, tank AOE snap threat isn't what it used to be (comparatively) so even well-geared ones can't handle immediate threat explosions.
For pugs or tanks whom I don't know, I've started to IT/PS/Pest the primary target, even as frost, in order to give the tank time to build up tertiary threat on the non-focus mobs. Once the tank has build up a good rotation of AOE threat I can start Howling, if appropriate.
If the issues persist try IT/PS/Pest starters in Unholy presence. Last resort is to down-grade your weapons to ones with lower median damage (thus reducing the coefficient AP values). While fairly drastic, it can help you get through a pug where topping the meter is less important than smoothly completing the instance (badgers - stress > dps).
Good luck!