Sometimes MTU issues arise, particularly with PPPoE on Cisco DSL CPE. These can be addressed by changing the TCP Maximum Segment Size (MSS) on the pertinent interface:

interface Dialer1
  ! ...
  ip tcp adjust-mss <size in bytes>
  ! ...

When you're doing a packet capture for the purpose of examining the frame payload, you'll want to extend the snaplen (snapshot length) to 1515. That's long enough to accomodate the 1500 MTU and should give you a pretty good look at what you're after.

For example:

# tcpdump -s1515 -X -ieth0 -w sample.cap

Note: This applies to 'ethereal' and 'wireshark' but their defaults are to capture max(INT) by default.