I just tried this on c2075. The result agrees well with the c2050 card (107mH/s). There is fluctuation (from 70 to 140).
The fluctuation come from you "slow" card. The summery is build every 30 sec. It can be that some times the count goes to the next or previous intervall. A drop is also possible if you get no connection to the getwork server. The best is to use the
http://mining.bitcoin.cz/mining-proxy-howto stratun proxy. If you use them you get better and smoother results.
psychocoder