The bandwidth is used for the metric but also for other purposes like the load ratio in show interfaces.
In the IGRP calculation the minimum bandwidth along a path is taken so you have to make sure that you don't have any routes with bandwidths smaller than you T1/768k link. To make the unstable link less preferable configure it a smaller bandwidth (like in your example). The stable link should stay at T1 BW.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...