Don't forget capitalization - 5 Mb/sec is definitely not the same as 5 MB/sec.
10base2 isn't only 5 Mbps - it runs at the full 10 Mbps of standard Ethernet - but it's half duplex only. Twisted pair can be full duplex, depending on whether it's connected to a hub, a switch, or directly to the Ethernet of another computer.
It may be worthwhile to try 7.5.5 to see if the System software is not quite right.
10base2 isn't only 5 Mbps - it runs at the full 10 Mbps of standard Ethernet - but it's half duplex only. Twisted pair can be full duplex, depending on whether it's connected to a hub, a switch, or directly to the Ethernet of another computer.
It may be worthwhile to try 7.5.5 to see if the System software is not quite right.