Turboprop Autothrottle?

I have been looking/experimenting with adding an autothrottle to the Mike Stone C130. I have added the proper lines, switch and readout window to the .cfg and panel. When I engage the AP and AT, and set the proper IAS, everything works fine until I reach 155 IAS. The throttles then come down to idle. Is there an adjustment to the ACcfg that I have overlooked? Is there a turboprop AC with AT that I can use?

Thanks
Bill