PROVIDENCE CTLR EXPERIENCED LOSS OF SEPARATION WHEN A BE58 MAKES UNEXPECTED TURN IN HOLDING PATTERN DURING GPS APCH TO MTP; CONFLICTING WITH ANOTHER IFR ACFT ON VECTOR TO FINAL.

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ACFT X; BE58; WAS INBOUND TO MTP ARPT AT 5000 FT. ACFT X WAS INSTRUCTED TO PROCEED TO SEY VOR; THE INITIAL APCH FIX FOR THE GPS RWY 24 APCH AT MTP. ACFT Y; P28A; WAS NE OF SEY SWBOUND. ACFT X WAS INSTRUCTED TO CROSS SEY AT 3000 FT AND CLRED FOR THE GPS RWY 24 APCH. ACFT X REQUESTED TO PROCEED TO FEKSA; AN INTERMEDIATE FIX ON THE GPS APCH. THE PLT WAS THEN TOLD TO PROCEED TO FEKSA AND CLRED FOR THE APCH. I EXPECTED THE ACFT TO PROCEED OVER FEKSA; THEN STRAIGHT-IN TO MTP ARPT. THE ACFT DID AN UNEXPECTED TURN IN THE HOLDING PATTERN AT FEKSA WHICH WAS NE OF THE FIX. THE TURN TO THE NE PUT THE 2 ACFT INTO CONFLICT WITH EACH OTHER AND SEPARATION WAS LOST. SUPPLEMENTAL INFO FROM ACN 617808: THIS INCIDENT OCCURRED WHILE I WAS ON AN INST APCH; DURING A PROC TURN. IT WAS MARGINAL VFR/IFR. I WAS CLRED FOR AN APCH. THERE WAS CONFUSION BTWN MYSELF AND CTLR AS TO WHERE TO BEGIN THE APCH. I SPECIFICALLY TOLD HIM THAT A PROC TURN WOULD BE REQUIRED. WHILE IN A PROC TURN; MY TCASII WARNED ME OF AN IMMINENT COLLISION; WHICH REQUIRED EVASIVE DIVING TO AVOID A TARGET THAT PASSED VERY CLOSE TO MY ACFT. I HAD THOUGHT THAT THE AIRSPACE WAS FREE OF OTHER TFC WHILE ON THE APCH. I BELIEVE THAT BTWN THE CONFUSION OF THIS BEING A RELATIVELY NEW APCH AND A CTLR OVERLOAD; A VERY CLOSE NEAR MISS OCCURRED.