md5 diff after scp transfer

I am running a RHEL 3.0 on one machine and RHEL 4.0 on the the other -
both intel machines.

I have a cron that transfers a very large(2.8Gig AFTER gzip) file. I scp
the file from one machine (RHEL 3.0 - on a T1) to the other (RHEL
4.0-also on a T1), and find that their md5sums are different. However,
when I transfer the file over the local network(sames two machines -
just in the same building this time), the files match. It seems to have
something to do with an internet transfer only.

Questions:

1. How to I troubleshoot what is happeneing and when? Logs? Run scp in
debugging mode? The file is clearly coming over corrupted...

2. Can I use scp or rsync with md5 during the transfer (as opposed to
after the transfer) to ensure that it gets over clean? If not, what then?

Also - station wagon transfers are not an option, thanks.


Thanks -D