What you're describing is known as Warfarin Failure. Do a search for this term on:
http://asheducationbook.hematologylibrary.org/cgi/content/full/2001/1/339

I have had 2 events the second caused me to have a bracial arterial bypass. was put BACK on coumadin. with in 6 months that bypass was blocked. we continue to watch and see my blood flow from my other outlets decrease. most likely i will be having another bypass soon. i am on coumadin for the rest of my life. would love to see the research that is being done on medication that is not so harsh become a reality.

while you may think that you are theraputic and taking warfarin, you have to know that if you are taking "warfarin" and not the brand name of coumidin, you have a chance of not being properly medicated with the generic brand because of the fact that they only need to be 80% accurate with their dosage. I have had experience with getting a clot after taking a "new" dose of the generic brand and after changing back to the brand name, my pt ended up being back to where it should have been.