Volvo Repair: GREMLINS UNDER THE HOOD, flash flash flash, example 121

Question
HELLO,

MY DAUGHTERS 1990 740 W/230 ENGINGE HAS A MYSTERIOUS PROBLEM THAT COMES AND GOES.  SOMETIMES AT IDLE IT WILL START RUNNING ROUGH AND THEN DIE. ONCE RPMS ARE ABOVE IDLE IT RUNS GREAT.  FIRST TIME I CHANGED PLUG/WIRES. RAN GOOD FOR 2-3 DAYS THEN PROBLEM RESURFACED. GOT UNDER THE HOOD AGAIN CHECKED FOR VAC LEAK, CHECKED WIRING AND THE ENGINE RAN GREAT AT IDLE, BUT I REALLY DIDN'T FIX ANYTHING. 2-3 DAYS LATER SAME PROBLEM. EVERY TIME I GO UNDER THE HOOD I SCARE OFF HTE GREMLINS AND TWO DAYS LATER THEY'RE BACK. THIS HAS BEEN THE STATUS QUO FOR 2 WEEKS AND I CAN'T NAIL DOWN THE PROBLEM.  HOPE YOU CAN HELP. THANK YOU JIM BRUGHELLI  

Answer
first I would check for codes in the control units.below is how to do  so.in the engine bay on the drivers side near the strut tower there is a small black box with a wire attached to it.place the wire in the terminal marked 2.turn ignition key on (not started)press the small button for 1 second and the code will be displayed as a series of flashes.a red led displays the codes.the code will be three digits.small pause in between flashes for a single number long pause between flashes for the next number.example 121 will be flash.....flash..flash.....flash.repeat three times or until first code repeats itself.to erase the code after all codes have been checked hold the button for a good 5 seconds then release the light should light steady,then press for another 5seconds.this will erase the codes.  email me the codes and i will tell you what they are..socket 2is the fuel system 6 is ignition.