RACH overload is a load on the Air interface, not the Abis interface. It is alcatel B9, isn’t it ?
If the RACH is overloaded it doesn’t mean your RSL on the Abis is overloaded. You should check the Abis congestion instead.
But if you think the RACH is congested, then you should check your S and TX_INT parameters. Are they set at the correct values ?
You can activate the immediate assignment reject feature, to avoid RACH “channel requests” repetitions.
You can perhaps play with AGCH queue parameters (if this is alcatel, then you don’t have access to those, i think)
And perhaps your RACH is overloaded due to GPRS ? Can you check the number of PS channel requests and the number of CS channel requests ?