AN SMT FLYING IN ZSE; WA; AIRSPACE ENCOUNTERS TURB AND FINDS IT DIFFICULT TO MAINTAIN ALT SEPARATION WITH ANOTHER ACFT IN A HOLDING PATTERN.

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WE WERE CLRED FOR THE ILS APCH AT PAINE FIELD. WX WAS 2000 FT BROKEN; 4000 FT BROKEN; 7000 FT OVCST. WINDS OUT OF THE S AT 20 KTS; GUSTING TO 33 KTS. WINDS ALOFT WERE AS HIGH AS 40-55 KTS. WE WERE CLRED TO 3000 FT AND WERE TOLD OF AN ACFT IN A HOLD AT NOLLA NDB AT 4000 FT. WE WERE IMC AND HAD NO VISUAL CONTACT. WE WERE POPPING IN AND OUT OF CLOUDS AND WERE EXPERIENCING MODERATE TO SEVERE TURB WITH LIGHT MIXED ICING. DUE TO TURB; OUR ALT WAS FLUCTUATING +/-400 FT OR SO. WE WERE TRYING TO KEEP THE ACFT IN CTL MORE THAN ANYTHING. AS WE TRACKED THE LOC INBOUND; WE WAITED FOR A GS INDICATION. THERE WERE NO FLAGS BUT THE INDICATIONS WERE UNLIKELY (IT SHOWED US BELOW GS WHEN WE SHOULD HAVE BEEN DSNDING). WE WAITED A BIT TO TRUST OUR INTUITIONS; BUT WITH THE ALTDEVS DUE TO TURB AND BECAUSE OF OUR NOT DSNDING ON GS WHEN WE SHOULD HAVE (INST ERROR); THE CTLR'S ALARM WAS SET OFF (DUE TO OUR PROX TO THE ACFT IN THE HOLDING PATTERN) AND WE WERE ASKED TO CALL CTR WHEN WE LANDED. WE ATTEMPTED THE APCH ONCE MORE AND THEN FLEW A VISUAL APCH IN TO BOEING FIELD (UNEVENTFUL). GS WAS STILL INDICATING ERRONEOUSLY AND WITHOUT A FLAG. FREQS IDENTED PROPERLY. UPON CALLING CTR; WE WERE TOLD THAT OUR MODE C WAS INDICATING US TO BE 600 FT OFF OF OUR ASSIGNED ALT. OUR INDICATED ALT (WITH CORRECT ALTIMETER SETTING) WAS OFF BY A MAX OF 400 FT IN THE WORST TURB; AND THE ALT FLUCTUATIONS WERE RAPID. IT WOULD HAVE CAUSED UNDUE STRESS TO CHASE THEM IN SUCH TURB. WE WERE ALWAYS IN CTL OF THE ACFT BUT WE WEREN'T ACCUSTOMED TO HAVING SO MANY ADVERSE THINGS COME UP AT ONCE.