Air Traffic Control Fully Certified
Other / Unknown
ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
General None Reported / Taken
ATC Equipment / Nav Facility / Buildings
I was working a combined position with sector 10; 11; and 12 combined at 12.  At approximately XA50 local I got the handoff on Aircraft X a ZZZ arrival at FL390 that was in cruise; but had to be descended beneath two aircraft Aircraft Y at FL360 and an Aircraft Z aircraft at FL380. I issued Aircraft X a crossing restriction to miss both pieces of traffic; 'Cross OTT at FL350'; but later corrected this to 'Cross 10 miles south of OTT at FL350' to over separate Aircraft X from Aircraft Y and Aircraft Z.  Aircraft X's pilot reported out of flight level 390 and was started his descent.   I rogered this and continued my duties. Over the course of my next coupe of scans and saw that separation was obtained for Aircraft X with Aircraft Y and Aircraft Z as Aircraft X leveled at FL350 complying with the restriction I had given him.  However; for his descent I used a interim altitude and could see his mode C altitude.  Between OTT and PALEO; where the suspected loss occurred; Aircraft X's reported Mode-C altitude varied between 350 and 351.  This is a common occurrence and I disregarded it was a equipment malfunction.  When Aircraft Y; still at FL360; got within 5 miles of Aircraft X; Aircraft X's reported Mode-C jumped to 351 and caused a suspected loss of separation which I only became aware of after reviewing the FALCON replay.  I did not verify Aircraft X's altitude to see if he was level at FL350 or if it was an equipment malfunction.  I cannot make a recommendation for this event because I feel my procedures were correct and the restriction I gave to Aircraft X ensured separation with both of the conflictions.  The fluctuating Mode-C altitude; in my opinion; was the cause of the separation error and it just a fault of the system between the transponder on the aircraft and the receiver on the ground.
Enroute controller reported a possible loss of separation but only after the suspected conflict was identified after a reviewed on the FALCON replay system.