Help diagnosing PPTP throughput issue.

Have an Intel Mac 10.6.5. When I Remote Desktop via PPTP to a W2K3 server my throughput is atrocious. When I VPN using same credentials and network via a Slower Vista Business machine everything is speedy (10x faster). Using Wireshark to attempt to determine issue. Fixed a small MTU mismatch and get these occasionally in the trace (maybe 10-15%)

[TCP Retranmission] [TCP segment of a reassembled PDU] AND [TCP Dup ACK etc.....]

Are these normal? if not what do I look at to correct, if they are normal any suggestions on what else may be culprit and how to track down.

Thanks for any assistance!