From some networks requesting certain images on our webserver is very flakey. I've looked at tcpdumps on both sides and the server sends back part of the file and the client ACKs the TCP packet but the server never receives the ACK.

The servers view:

41  19.941136   212.169.34.114  209.20.73.85    TCP 52456 > http [SYN] Seq=0 Win=8192 Len=0 MSS=1460 WS=2
42  19.941136   209.20.73.85    212.169.34.114  TCP http > 52456 [SYN, ACK] Seq=0 Ack=1 Win=5440 Len=0 MSS=1360
46  20.041142   212.169.34.114  209.20.73.85    TCP 52456 > http [ACK] Seq=1 Ack=1 Win=65280 Len=0
47  20.045142   212.169.34.114  209.20.73.85    HTTP    GET /map/map/s+74-WBkWk0aR28Yy-YjXA== HTTP/1.1
48  20.045142   209.20.73.85    212.169.34.114  TCP http > 52456 [ACK] Seq=1 Ack=522 Win=6432 Len=0
49  20.045142   209.20.73.85    212.169.34.114  TCP [TCP segment of a reassembled PDU] (Part of the content of the image 2720 bytes. i assume it is reassembled in tcpdump and it is fragmented over the wire.)
** never receives the ACK sent in frame 282 and will eventually resend the tcp segment **

The clients view:

274 26.161773   10.0.16.67  209.20.73.85    TCP 52456 > http [SYN] Seq=0 Win=8192 Len=0 MSS=1460 WS=2
276 26.262867   209.20.73.85    10.0.16.67  TCP http > 52456 [SYN, ACK] Seq=0 Ack=1 Win=5440 Len=0 MSS=1360
277 26.263255   10.0.16.67  209.20.73.85    TCP 52456 > http [ACK] Seq=1 Ack=1 Win=65280 Len=0
278 26.265193   10.0.16.67  209.20.73.85    HTTP    GET /map/map/s+74-WBkWk0aR28Yy-YjXA== HTTP/1.1 
279 26.365562   209.20.73.85    10.0.16.67  TCP http > 52456 [ACK] Seq=1 Ack=522 Win=6432 Len=0
280 26.368002   209.20.73.85    10.0.16.67  TCP [TCP segment of a reassembled PDU] (Part of the content of the image. Only 1400 bytes.)
282 26.571380   10.0.16.67  209.20.73.85    TCP 52456 > http [ACK] Seq=522 Ack=1361 Win=65280 Len=0

The network we are having trouble with is NATd. Is there any kind of explanation for this weirdness?