AN ACR B737 WAS CLRED TO CLB TO FL290 AND EXPECT FL330 LATER. ARTCC CTLR LATER ASKED THE B737 HIS ALT AND THEN DSNDED HIM TO FL280 AND TURNED HIM 15 DEGS FOR TFC. LTSS OCCURRED SHORTLY THEREAFTER AND THE RPTR ASKED FOR PHONE NUMBER TO CALL THE ZOA UPON LNDG. CALLBACK SHOWED THAT A SYS ERROR HAD OCCURRED AND THE CTLR WAS AT FAULT.

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IN CRUISE AT FL280 20 MI W OF CZQ ZOA INSTRUCTED US TO CLB TO FL290 AND EXPECT FL330 LATER. AFTER CLBING TO FL290 AND LEVEL FOR APPROX 5 MINS; ZOA DEMANDED WHAT ALT WE WERE AT. AFTER REPLYING FL290 THEY THEN INSTRUCTED US TO DSND TO FL280 AND TURN R 15 DEGS IMMEDIATELY. WE SAW TFC AT 12:00 O'CLOCK POS; 4 MI AT FL300 XING L TO R. WE TOLD ZOA THEY HAD INSTRUCTED US TO CLB TO FL290 AND EXPECT FL330 IN ORDER TO CROSS THE 'COMPLEX' IN OUR READBACK. ZOA REPLIED THEY HAD INSTRUCTED US TO MAINTAIN FL280. I ASK FOR AND RECEIVED A PHONE NUMBER FOR OAK AREA B TO CALL ON THE GND AT LAS VEGAS. I LATER CALLED AND FOUND OUT THAT WE HAD BEEN CORRECT IN OUR READBACK AND THE CTLR WAS AT FAULT. THEY THANKED US FOR OUR QUICK AND PROMPT ACTION. BEFORE THE QUESTION OF BEING AT THE RIGHT OR WRONG ALT WE DID SEE THE TFC ON TCASII AND WONDERED A LITTLE WHY WE ONLY HAD APPROX 1000 FT CLRNC WHEN BOTH ACFT WERE ABOVE FL280. I MAYBE COULD HAVE ALLEVIATED THE SIT A LITTLE BY QUESTIONING THE CTLR A MIN OR TWO EARLIER. TO US AS A FLC THERE SEEMED TO BE ENOUGH SEPARATION VISUALLY BUT FOR ATC STANDARDS I DON'T THINK IT WAS ADEQUATE. I WILL CONTINUE TO READ BACK ALL ALT CLRNCS CLRLY AND QUESTION PROBABLE CONFLICTS MORE OFTEN.