Thanks rizvanrp. My wife is using Cisco VPN ver 4.8.01.0300 and XP Professional SP3 and like what you said I also think it is the problem with DIR-615 as she has never faced this problem before when she used streamyx at my parents' and also in-laws' house during balik kampung. is there any other setting that I can change in DIR-615 to test ? I already change the wireless mode to B/G only but it doesn't solve the problem.

Will try to use wired connection today to test but she still need to use wireless as her "office" is at upstairs but the router is in the lviing room. if I can confirm the problem is coming from DIR-615 I will complain to Unifi.

you will not lose VoIP as it is not handled by the DIR-615

Well, this is really a process of elimination so you'd really want to remove all possible variables to ensure that it is infact the DIR-615 G1. One of these variables would be the wireless link, maybe there's a device operating on the same frequency range causing interference at certain intervals.. who knows ? Best way would be to use a wired connection to ensure that this disruption is happening at the DIR-615 G1 <-> TM's side.

If possible, log into the DIR-615 G1 and ensure that the LAN to WAN and WAN to LAN firewall are completely disabled. I've seen these settings interfering with IPSEC site-to-site VPNs on Unifibiz setups and even the TM staff couldn't figure it out

If this doesn't help, I really doubt TM will be able to help you. Some of their techs aren't aware about how their own hardware works because VLAN tagging is all performed in the background and hidden from the web interface. Not to mention the whole web interface is pretty much locked up and advanced options are kept away from being modified.
These are some of the advanced options available to DIR-615 G1 dd-wrt flashed systems that might help out with your VPN situation.

@eddie_lim

You won't lose VOIP because VOIP is handled by the Fiberhome. You lose IPTV because the firmware I used to reflash the DIR-615 G1 does not completely support the new switch chip and cannot allocate the IPTV port to a separate network interface.. so there's no way to separate IPTV + internet traffic if you enable VLAN 600 on it.

You can use a Linux box with 3 hardware NICs, then have it act as a machine to bridge the VLAN interfaces to physical interfaces (thus removing the VLAN tagging). I'd do this but using a full desktop just to strip VLAN tags is kind of wasteful, would rather just use a VLAN aware switch/router to do it (less power consumption).

I wish TM would have just used no VLAN settings for the PPPoE connectivity, at least everyone would have a choice on what hardware they wish to use. Unifibiz (corporate) users are basically going to get screwed over because they're given the exact same hardware and setup as home users. The only difference is they're allocated 2 static IPs. If their hardware firewalls don't support static PPPoE connectivity over VLAN interfaces, they're going to regret switching over to Unifi.

@defari

Haha, like I said.. it's the Unifibiz corporate users who are going to get screwed over here. With a VIP20 line they expect to serve >100 nodes at once, no way this router can handle that connection load easily for a long period of time. Can't even set up VPN links properly since it doesn't handle port forwarding correctly. Stock firmware is pretty crappy, they're better off flashing to DD-WRT since they don't need IPTV support.

Btw, they released new firmware for the dir-615 g1? Any idea where to pick this up?