2000 Buick LeSabre with intermittent misfire

My wifes 2000 LeSabre (3.8 v6) has a wierdly intermittent misfire. Sometimes it has miisfired like crazy for 5 minutes when started, then wont misfire again for a week. Sometimes it has not misfired in 2
weeks then will misfire slightly whist cruising up a slight incline on the expressway with a very slight amount of gas. If it is driven gently it seems to misfire more than when it is driven hard.
I replaced the plugs and wires because it had 70k miles on it and this did not fix it. The old plugs came out looking perfect btw.
The misfire will set the check engine light, but the next day it goes out when the misfire dissapears.
I was going to get a scan cable and software from http://www.obd-2.com/ and plug it into a lap top and run around for a week or two.
Anyone have any experience with http://www.obd-2.com/ hardware and software?
Also, any one any experience on what such an intermittent problem might be?
Thanks for any help.
Bob
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Even though the light went out the codes are still in the computer so have it read to find a pointer to the problem. I have used the cable from OBD-2.COM for several years and it works well. The software is actively upgraded and improved.....

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Thanks, I think I am going to get the cable from them since this problem is so intermittent.

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