I was going to reply along those lines, but didn't want to depress you! I had a revelation when my DS 1 was about 4 months. He slept for 40 minutes, to the minute exactly. No matter how much rocking or shhhing etc could get him back to sleep.

So I just realised that he wasn't going to have any more sleep and that was that. Life was so much easier then.

FWIW, he grew out of it himself around 8 months when he moved to two sleeps a day.