Hi, I bought the 720 about 6 days ago and faced similar blackout issues several times. A point to note is, If I call my mobile when it is in blackout or hanged state, I can still hear the ringing sound from the other phone I used to dial the 720 but no ringtone from it!
If we are told to avoid multitasking by turning other apps off, then I,feel its very similar to what we used to do with Win98 OS in the 2000s! I am a Nokia fan as well, I think Nokia has to do,something very fast to tackle this and definitely not suggest an upgrade to 925 or other models!