its an old Narco AT150 TSO Mode C.
Some, not all, instances of this particular transponder do not play well with modern Mode-S interrogators. In the UK this has generally been seen as the aircraft returning a Mode C at about FL190 when the aircraft is actually around about 3000 feet. When the UK still had Mode A/C interrogators as well as Mode-S the aircraft would be shown at the correct height for controllers using those radars and at the incorrect height for those using the newer radars.
It is probably your transponder but you will probably find when checked out in the shop they can find nothing wrong with it because it can meet all its original specifications and still cause this issue. There are others who frequent this forum who know far more about this particular issue than I but I believe it may have had something to do with the transponder sending a Mode A reply in response to a Mode C interrogation.