abscessed tooth resistant to antibiotics

I have a cracked tooth (with a crown) that has become abscessed (lower right molar).  The abscess first occurred about 4 months ago and was treated successfully with a 10-day course of Erythromyacin.  Then the abscess came back about one month ago.  I have since tried to treat it with a 7-day course of Moxycyllin (unsuccessful) and now a 10-day course of Clindamycin.  It doesn't seem like this course is working either.  My endodontist seems to believe that the antibiotics will work eventually.  The abscess hasn't been drained yet.  It is not very painful, but mildly uncomfortable.  I have had no fevers or other discomfort in my body or face.
Once the abscess is cleared up, I am supposed to get a root canal on this tooth.  Do you have any advice on my best course of action for a solution?  Thank you.

[scottma]

If the infection source is from dental pulp, root canal treatment should be rendered as soon as possible.

[tmborden]

Thanks.  Can a root canal be safely and adequately performed while the tooth is still infected/abscessed?

[tmborden]

Thanks.  Can a root canal be safely and adequately performed while the tooth is still infected/abscessed?

[scottma]

When root canal treatment is delivered, the abscess will be drained.the infection will clear up very fast.