That last speedtest result has a very good download speed. The upload speed could be a little better though. That result shows for ADSL2+ you must be either close to your exchange and have reasonable cabling to your house. This shows what your line speed is capable of so Optus should be able to stabilise this for you.
I had a similar experience with Telstra Bigpond and knew what speeds I should have been getting. Turns out in my case that they reconnected me for some strange reason to a faulty DSLAM port at the exchange. After hounding them I was reconnected by a techo to a decent port...problem fixed?
Be a squeaky wheel and stay on their case Dennis.
Good luck.