The key to success on a busy channel is short packets:
N3YKF-3>APTT4,ARISS,IS:/131918z2901.30N/08210.41W>288/000/n3ykf mobile!/A=000121!wgO!
There are 41 bytes in that packet that are of no value in this application.
1) Remove the path "IS" (is 7 bytes in the actual protocol) and it is not valie anyway for APRS
2) A time stamp is usually not needed, since APRS is always LIVE, it is valid the instant it is reecived
3) No need to repeat your call in the packet since it is in the callsign field
4) No need to say "mobile" because you are using ths symbol for a car anyway
5) When going through the iss at 7,000,000 feet high there is not much value in reporting your altitude as 121 feet.
6) When making a thousand mile contact the PRECISE position data in the !wgO! format to 6 inches is not needed
7) Use the !DDMM.mmN/DDDMM.mmW>... format without the time stamp.
8) Make sure your TXD is 150 or 200 ms instead of te default.
That should cut your packet timeto HALF or more and double your chance of success.
Bob, Wb4APR