Perhaps the 10/100 gear and the Gigabit equipment is not 100% compatible at extreme cable lengths.

Try inserting a simple 10mb or 100mb hub between the gigabit switch and the long cable.

Or get another gigabit device in place of the 10/100 unit to test with

Another test would be to manually force half duplex mode on the link. Try that at each speed.

Get a 300 foot cat 5e cable and carry the entire long cable and 10/100 end gear to the gigabit switch location. See if you can get it to work all in one place.