Black Hole Router

I am by no means a router/network guru so I am posting this here hoping someone can help me. I have a Nortel ARN Router connecting to multiple Nortel ARN Routers over a Frame Relay. All of my sites are fine except for 2 that are both going through a off net 3rd party. When I try to ping <ip address> -f -l 1472 it returns "request timed out". When I try to ping <ip address> -f -l 1473 it returns "packet needs to be fragmented but DF set". When I try to ping <ip address> -f -l 1018 it returns "reply from <ip address>". I don't want to lower my MTU on my host interface because that would screw up the rest of my network. And I'm not sure how to check to make sure it's not my routers. Is there a way to only send 1018 size MTU down just these routes and leave the other routes alone? Probably not but I thought I'd ask.

Also, what can I tell my 3rd party to check without sounding like an idiot.

I have also pinged from the routers with the same problem.

Any help would be greatly appreciated. Thanks in advance.

You could allow fragmentation which wouldn't affect those that have acceptable packet sizes and would allow those too big to be fragmented. This would add overhead to the 2 lower packet size links, but it would get there.