930 STALLING PROBLEMS

HELP..THIS IS MY FIRST TIME . I RECENTLY PURCHASED A 88 930 3.3.THE PREVIOUS OWNER CHANGED TO PERMATUNE IGNITION FROM CDI BUT RETAINED THE STOCK COIL.I CAN BE DRIVING ALONG AND HERE A BACKFIRE THROUGH EXHAUST THEN IT JUST DIES.WAITING A FEW SECONDS IT USUALLY STARTS UP.THIS ONLY OCCURS ABOUT ONCE A MONTH.I'VE OWNED THE CAR FOR ABOUT 6 MONTHS.I'VE CHECKED GROUND CIRCUITS FOR RESISTANCE VALUES AND PERMATUNES CHECKS FROM THE NET HELP ALL WITHIN SPECS.THE CAR HAS NEW FUEL PUMPS FRT.AND REAR WIRED ON SEPERATE CIRCUITS OF THEIR OWN AS SUGGESTED.I'VE DISCONNECTED THE FACTORY ALARM THINKING THAT THIS MIGHT HELP.I STILL HAVE THE FACTORY PLUGS THE COLD PLATINUM.OTHER THAN THIS IT RUNS STRONG WITHOUT A MISS.ANY REPLIES OR HELP WOULD GREATLY APPRECIATED.