First off, I don't know who is monitoring your coumadin but your INR--established by the World Health Organization--should be between 2.0 and 3.0. INR stands for International Normalized Ration. This means that no matter where you go or what instrument is used, the same method of calculation is used to determine the prothrombin time (how long it takes your blood to clot).
As for you INR being difficult to regulate, If coumadin levels were easy to maintain, there wouldn't be coumadin clinics. I've been on coumadin for four years now and still fluctuate in and out of range. Exercise, stress, illness, other medications; all these things affect how coumadin is metabolized.
Concerning vitamin K, my doctor didn't make me quit eating leafy greens. The trick there is to always eat the same amount on a regular basis. For instance, if you eat three salads a week, you have to eat three salads EVERY week. You can't eat two salads one week and one salad the next. That will throw your INR off. In other words, don't work your diet around your coumadin. Work your coumadin around your diet.
I hope this helps and good luck.
Duke University Medical Center recommended the target range of 3.0 to 3.5 for the St Jude Medical leaflet valve. I have been on it since 1993. The instability has been only the last three years or so. Here's a link to a presentation that suggests vitamin K and the logic for doing so; http://www.dcpa.us/slides09/sun05.trujillo.vitk.pdf