sendto Socket packet size

Hi. I posted a thread about jumbo frames a couple of weeks ago. Here is a better description of what is happening now.

1.) The mtu for my network adapter is set to 9000.
2.) I can type "tracepath <ip>" and it returns a pmtu of 9000 to another computer.
3.) My buffer is 153600 bytes. So I need to send 18 (8192 byte) packets or 37 (4096 byte) packets.
4.) The rate at which data is being sent is 153600*60/second...so lots of data.
5.) Using the sendto command, I can send packets of size 576, 1400, 4088, and 4096, (and anything less than 1400) without many problems.
6.) I cannot use 4000, 8192, 9000,....and lots of others, except the above, when calling sendto or the program cannot keep up with sending and gathering data.

Does anyone have any idea why the specific values work, but not the other numbers?

Thanks,
bj00