So you're cruising along at about 45 MPH under a light throttle and all of a sudden it feels like someone turned the ignition off and back on quickly????

That's most likely caused by a dirty/worn throttle position switch. You can carefully open them up and clean the printed contacts with alcohol. Some people say they've used an ink eraser also to clean them. I've had luck slightly tweaking the contact "fingers" so that they track across a fresh path when the throttle opens.

When you open up the switch, have an open rag under it as there are little rubber seals in the corners of the switch that may fall out.