NMap Scanning Issues

I am trying to run Nmap both on specific ports and then multiple ports (1-1024, 1-65535, etc). My issue or is this: when I run small port scans; my results come back fine (open, closed, filtered). I have confirmed that the results are accurate. However, when I attempt to scan multiple ports I get various unreliable results. For instance, a port that I could have confirmed either open or closed may show up as filtered or the opposite.

No configuration changes have been made and no firewall on remote site.

Additional Notes:
-I am running over the Internet
-I am running from behind a Netgear Firewall/VPN/Router (FW114) with NAT
-Cable/DSL Connection (Fast)

I am wondering if my router is timing out or perhaps I am performing a DOS of my own router. Does any have any thoughts or experienced this type of problem? If so, does anyone have any recommendations?

Thanks,
09Sparky

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