Truck speed drops 10 or so mph while cruising down the highway. Takes a few seconds to regain speed
We need more information. What engine do you have? Manual or auto transmission? Have there been any other issues lately, like other problems? Any recent repairs/maintenance? Does the speed drop while driving on flat surfaces, going up hills, or what?
Or is this cruise control related?
Difficult to tell, the OP’s statement is a bit vague, but I think he is talking about the cruise control’s action.
“while cruising” has several different meaning, but perhaps it is “while driving under cruise control” ?