Firstly, Asteroid is rubbish and really buggy I find, what your seeing with the fluctuations is normal for Asteroid as far as my experiences are concerned. I tested it on my Mac with my rig of Antminer U1's and it did the same. The latest version of Asteroid has the proper support for Antminers so it utilises the higher clock speeds automatically, but as you have found it goes up and down a lot. You should ditch Asteroid and install Bfgminer on your Mac for much more reliable hashing. There are plenty of set up guides for Bfgminer and the U1's.

My Antminers are all controlled via a Raspberry Pi now which stays on 24/7 and uses a LOT less electricity than leaving your Macbook on all the time. You should consider something similar as you're probably paying more on electric than your getting selling 2GH/s mining contracts. But on my Mac while testing Bfgminer with my U1's using the x0981 clock setting I get a reliable and constant 2.0 GH/s. When you first start Bfgminer and it starts mining, it's normal for the GHS to spike to very high, as much as 8 or 9 GH/s in some cases, or it may start really low with a few hundred MHS, this is normal and it levels off after a few minutes.

Secondly, you say you are selling mining contracts, with a single 2GH/s Antminer?! Are people actually silly enough to pay for 2GH/s mining?!