What type of traffic and how is it configured? I have seen sequence problems with Multilink.
Do you have the ppp multilink command on the s0:15 interface as well as the dialer? if not, add it.
What can happen, particularly if bridging over the link is that two packets are sent from the source a big one then a little one.
The big one goes down the first channel, then the second one starts down the second channel. That completes at the destination first and can be forwarded ahead of the other packet. Depending on the traffic, this can be a major problem - ISO transport can have very serious problems with out of sequence packets, and ECMA is worse as an example.
The output of a sh ppp mul may be interesting.