I also had problem with this issue when using Opera 12 and earlier versions. On a computer with Windows Vista it was terrible. After I buy a new computer with Win 7 the problem with freezing webpages was better but did not disappear completely. I think the problem is more frequent on some webpages than other. So, maybe the reason for freezing webpages is how the pages are build and that they do not "like" the presto engine.
After I installed Opera 20 the problem with freezing webpages is gone. So I recommend you to try the new browser. It is simple, fast and I have not had any kind of problem with Opera 20.
Yes, you are right. Quite often me too got same problems. That some pages freeze Opera once it start. Then I just removed the page or only the image that freeze Opera. After that every thing worked perfectly. But........ now I even checked all my pages into Opera and I just couldt found any thing wrong there. Now its working fine. I hope it will continue further? But as I am only using Opera than all other Internet Browsers. I have to know about the problems that can freeze Opera. I will be back to get some more support if problems will begin again. I dont think so since all is OK now. But I am still afraid that maybe it will happen later or sooner? But hope it says as good as it now.