@timatleeTry turning the PFS key group on P2 to off and see what happens. I have a couple of IPSec connections with Fortigates, 1 with 4 SA's but that one has PFS key group set to off. Unless I am mistaken, by default, the DH for P2 inherits the DH from P1 unless specified differently.
I also set my time lifetime 10% higher than the FortiGate, which seemed to help a lot.