If you've got a temp problem, you don't want a lid. I think that's why Roka was asking... probably assumed like me that you had a tight glass lid that wasn't letting any water evaporate off and cool things down. Sounds like if you just add a little clip on fan, you'll be OK.
I have to admit though... a 2 degree swing for 260W on a 55g seems like more than I'd expect. I've got 200W on a 46g, and only see a 1 degree bump or so. What size heaters do you have and how many? Sometimes if you don't have your heaters placed well, or have two of them that are set at different temps and can't tell if they're on or not - this will create temp swings in itself. Did the tank hold a pretty constant temperature before you set up the lights? Just wondering out loud if it could be something other than your lights causing the drift.