ATC Center Controller reported an aircraft was experiencing intermittent loss of engine power; but was able to land safely at their destination airport.

Narrative

Aircraft X; a BE50 flying at 13;000ft; called me to request a decent to 9000 feet because he had some kind of problem and was losing power. I issued a pilots discretion descent to 090 and to let me know what else I could do for him. The pilot acknowledged the descent and advised that all of his indications were perfectly fine but we're losing power and he would let me know. I advised him that the ZZZ airport was at his 10 to 11 o'clock and 4 miles. He acknowledged that and said he would be looking at the map to see what he wanted to do. I notified my CIC (Controller in Charge) that I had an aircraft losing power and not able to maintain altitude. The CIC came over and I provided them with the information that I had. A minute later I issued the ZZZ altimeter to Aircraft X and gave him an updated location of the airport relative to his aircraft. He rogered that and explained that he was doing better now since he left 130 and wondered if it might have been wake turbulence. I did a quick traffic scan of my sector and the one above me and found no aircraft that could have caused him wake turbulence. He replied that he had everything in the green and no indication why he would have lost power. I said that when he leveled off at 090 he would trouble shoot some more.Once level at 090; the pilot reported that everything was perfectly fine and there must have been something weird in the air there at 13 thousand feet. He added that is he didn't know better; it seemed like intake ice but there was clear air there. He reported that he was making all the power he was supposed to at 090. I asked him to describe the exact conditions that he experienced. He told me that the airplane stopped flying; the airspeed dropped off the nose of the airplane came up then dropped and they were not able to maintain altitude or flying speed. Engines indicated that they were at full power. Fuel pressure; oil pressure...everything was in the green. I suggested the possibility of wind shear and he agreed that that was the most probable explanation. I looked at the wind display on the URET (User Request Evaluation Tool) and it only showed a 7-knot difference and no direction change near his location. Not enough to account for this dramatic loss of airspeed. I called Aircraft Y and asked the pilot if he had a minute and then reviewed the incident with his. He was baffled too and could not come up with an explanation for the loss of airspeed. I asked Aircraft X if he had any turbulence and he replied that it was smooth; not a bump. He told me that the wind was about 63 knots at 090 but were almost 80 kts at 130. He said that the only explanation was the wind because the motors were completely happy; not a hiccup. After a couple of minutes; I wanted to make sure that the early decent and head wind wasn't going to affect his fuel consumption; so I asked him if he was good at 090 or wanted to climb back up for the rest of his flight to ZZZ2. He said that he didn't want to climb and was fine at 090 but that he had just experienced a loss of 35kts. At this point he was coming up on the boundary with ZZZ Tracon; so I called them and explained everything that had occurred with Aircraft X. I asked if they had any reports of wind shear or turbulence in their sector and the reply was negative. I let the pilot know and shipped him over to ZZZ Tracon. Before he left; he informed me that he had just lost another 20 knots in an instant. Soon after; Aircraft Z called me and said that they had been discussing the incident and suggested it could have been pitot tube icing. That made sense so I called ZZZ Tracon and asked the controller to relay to Aicraft X that it might be pitot icing. He told me he would. A few minutes later I was relieved. I took a meal break and then went to our safety office to report the incident. No one had an explanation for the reason that happened. We watched the FALCON (Radar Display Replay) replay and observed Aircraft X go from 115 to 72knots in about 75 seconds. Our playback matched what his instruments indicated so it wasn't an iced pitot tube giving erroneous information. I asked to safety office to try to contact the pilot so we could let him know this new information to try to help him solve the issue in case it was mechanical. The office called and left a message for the pilot to call ZZZ Center as soon as he could. At the time of this writing; the pilot has not called. We do know that he landed at ZZZ2.