Facing problem in bridge mode

[hadi57]

hi

i have 7 pfsense  boxes, some are are connected to telephone company as isp, others are conneted to other isp's, my problem is:

1- when configuring my dsl modem in bridge mode to connect to telephone company for internet i face slow connection, and facebook wont open, yahoo and msn massengers wont connect, i tried 1400 in MTU but no luck.
but if i configure the modem to connect in ppoe mode, connection will be very good, but i loose the benefit of connection from wan (dyndns).

2- i am not facing this problem with other ISP's using the modem either in bridge or in dsl mode.

help appreciated