I sent them a message to check the ohm range spec and it should be the correct 78-10 ohms. The only thing I can think of, currently, is to ask if the sender and gauge were tested through the entire range before installation? Maybe the rheostat winding has some open "dead spots" where the ohms vary. I have found on the Taiwan senders, that the armature can be very loose and have no secondary ground connection for said armature.
www.morrisgauge.com