Now I successfully switch to ADSL2+ and am back online again. The new one is spectacularly fast so that I may reach data limit (15 GB) so soon. However, it's still slow browsing Thai pages that their servers are in Thailand. I don't know why; may be because of the bottle neck of Thai gateway (port 80) or problem with the ISP proxy itself. Nevertheless, other kinds of data transfer, such as multimedia streaming, are great. I can watch most of Thai free TV via KU website smoothly, except MCOT; it's crappy slow -_-'.
In conclusion, switching to ADSL2+ is very great and cost effective at around 1,500฿/mo (it costs AU$49.95 and includes 500 min free calls from VoIP to capital cities in Aus and a great rate for international call). I wish I will have this in Thailand when I go back.
PS. I built Firefox 2.0 RC2 with Thai icu word breaking and uploaded it here.