Stevie Nicks [ tickets ]’ ongoing bout with a respiratory problem has resulted in the postponement of two more California tour stops. The singer has now postponed a total of six concerts in recent weeks due to the illness.

Nicks’ Tuesday and Wednesday night (8/21, 22) performances at Universal Amphitheatre near Los Angeles have been postponed. Make-up dates will be announced soon, and tickets for the original dates will be honored at those shows, according to Nicks’ official website. Refunds are available at the point of purchase, according to the venue box office.

Last week, Nicks postponed her Aug. 14 and Aug. 15 California engagements. Those performances have since been rescheduled for Sept. 30 and Oct. 1, respectively, according to Nicks’ website.

Last month, Nicks postponed her July 24 stop in Virginia and July 25 stop in North Carolina. Those dates will be made up on Sept. 4 and Sept. 5, respectively.

Nicks is expected to resume touring as scheduled on Friday night (8/24).

Despite the recent troubles, Nicks’ tour in support of May’s “Trouble in Shangri-La” (Reprise) has been one of the summer’s most successful outings. Pollstar’s latest breakdown of the top-grossing acts on the road right now places Nicks at No. 11, and attributes her last 12 shows with an average gross of about $470,000 and average ticket sales of 10,320 per night.

Nicks recently announced plans to give a Dec. 6 benefit concert at Phoenix’s America West Arena, one that will also feature guest performers. Proceeds from the event will support the Arizona Heart Institute Foundation’s cardiovascular disease research and education programs, according to a press release. A similar benefit that Nicks gave last year featured appearances by Sheryl Crow, Don Henley, Chris Issak and Nicks’ former Fleetwood Mac bandmate Lindsey Buckingham.

Premium seats to the benefit, priced at $1,000, are on sale now; a premium ticket gives one access to a private, post-concert party that Nicks will attend. Regular tickets will hit the box office on Sept. 29. More details are available at Nicks’ official website.