Aircraft leveling off and overspeeding with IAS/MACH mode engaged

[SteamingSpoon]

Hello,

Been enjoying flying the TriStar but it seems that every time I select IAS and MACH on the pitch mode selector during a climb, the autopilot/FD initially holds the desired speed but then pitches down, causing the the aircraft to accelerate past the speed target and overspeed if I don't intervene with V/S. Over two flights this seems to have happened consistently in both IAS and MACH hold modes. At this point it seems that V/S is the only appropriate vertical mode that can be used for a climb - I can't imagine that this is realistic at all. Has anyone else experienced this?