I had an 87 Suburban with 350- TBI. It did similar things- ran great idle, bogged down under a load. I discovered a small worn place in the insulation of the wires going into the carb throat for the ingectors. When under a load the engine rotated more (also worn engine supports) and the air intake horn rotated and shorted out the wire just enough to stop the injectors. A patch on the wire fixed it.