Agree with Waldy's summation above about trash mobs and dot clipping. Affliction locks just don't do well with trash mobs unless they are aoe-worthy.
I use a 6 spell rotation for boss fights that I cribbed from somewhere, but I can't remember the source now. It's the best I've found.
If I am in position before the start of the fight I lead off with Shadowbolt, Haunt, Unstable Affliction, CoA, Corruption, Siphon Life. If I lead off with SB and Haunt I have Shadow Embrace stacked twice which increases the output from the rest of my spells. With cast times and GCD I'm ready to cast my next SB right after Siphon Life, but I have to insert some pauses to avoid clipping dots after that. A good dot timer is useful. Since all these dots are on different timers I get some odds gaps where everything is up. I Spam SB in these gaps.
If I'm having to run into position at the start of the fight I lead off with the instant cast part of the rotation: CoA, Corruption, Siphon Life, Shadowbolt, Haunt, Unstable Affliction. I lose the benefit of Shadow Embrace for the first half of the rotation, but it's usually better than waiting to start casting until I get into place. Also, this rotation doesn't wrap around as neatly as the other one, and it's a bit of a scramble to keep all the dots up early on. Usually something in the fight distracts me enough that I can't keep all the dots up (adds, bad ######, etc) and then I just have to reload them all, which lets me revert back to the first rotation.
Aggro management is an odd thing to get used to as Affliction. Getting the 90% threat warning should be translated as "put your finger over your Soulshatter key and be prepared to press" because it's too late to stop the dots.