It almost is too smart for its own good. You need all 4 pair to get a 1g connection. It needs the pairs on pins 1,2 & 3.6 to work no matter what. If only those pairs work it will run at 100m. If either of those is bad it won't come up at all and what is really strange is if you have 3 pair working out of 4 it generally will not come up even if the 2 pair 100m needs are good.
I would cut another pair and make sure you only have the 2 pair working that 100m needs.