When it's lagging, check system/apps/running. How much free ram?
I hate to open up a can of worms unless its time to fish or cut bait (lying, worms are yummy) but I have found task killers to be useless. I've found some success with programs like system tuner or auto memory manager that tweak the built in memory management to free up more ram. Amm is a bit more newbie proof but I haven't tried it on the nex7.
If you need to stop or disable a rogue app, it's more effective to do it from system/apps imao.
My nex7 isn't lagging, so you're the guinea pig.