GLIDER PLT EXCEEDS 'WAVE WINDOW' GRANTED BY ATC.

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TODAY WAS A GOOD SAILPLANE FLYING DAY WITH HIGH ALT LENTICULAR CLOUDS OVER THE MINDEN TAHOE AIRFIELD IN NEVADA. I TOOK OFF AND REQUESTED THAT THE FBO TELEPHONE RENO CTR AND REQUEST THAT A WAVE WINDOW BE OPENED ABOVE 17999 FT. PERMISSION WAS GRANTED UP TO A CEILING OF 27000 FT IN THE 'W WAVE WINDOW.' AT 24500 FT MY ALTIMETER CEASED TO READ ANY FURTHER INCREASE IN ALT. I REASONED THAT I HAD REACHED THE MAX HT OF THE ASCENDING AIR MASS. I DID NOT DETECT THE MALFUNCTION FOR SOME TIME; UNTIL I CHKED A BACKUP ALTIMETER; AND DISCOVERED I HAD INADVERTENTLY CLBED TO 30050 FT. I IMMEDIATELY INFORMED THE FBO ON 123.30. THE FBO HAS TELEPHONE CONTACT WITH RENO CTR. I THEN BEGAN A CTLED SPOILER DSCNT TO BELOW 27000 FT. I DO NOT KNOW WHAT CAUSED THE PRIMARY ALTIMETER TO CEASE WORKING. IT BEGAN TO WORK AGAIN AS I DSNDED BELOW 23000 FT. MY GPS BASED CAMBRIDGE 302 FLT COMPUTER READ THE CORRECT ALT; AS DID THE BECKER XPONDER; BUT UNTIL I SUSPECTED THAT THE PRIMARY ALTIMETER HAD 'FROZEN;' I HAD THESE INSTS ON OTHER SETTINGS THAT DID NOT DISPLAY ALT. WHAT CAN BE DONE TO PREVENT A RECURRENCE? HAVE AT LEAST 2 ALTIMETERS VISIBLE AT ALL TIMES; AND PERIODICALLY CHK THE XPONDER FOR ALT; AS BACKUP.