Your speeds are low for a 5770. I have the same one and am using the same GUI as you (I wrote it, hehehe) and I get 164 MHash/s at stock clocks and 180 MHash/s when overclocked.
My guess is that you have some program running that is causing the GPU to downclock. For example, if I have Youtube open then my GPU clock goes down to 400 MHz and my hashrate plummets to 70 MHash.
If you have the Catalyst Control Center installed, you should give it a look at the GPU clock and see if it's 400 MHz, or if it's at the regular 850 MHz.
Hope this helps