1998 GMC Truck Hard shift into second from first

GMC K1500 with the 350 ci, and a throttle body. When my truck shifts from first to second, it shifts hard, like the throttle is sticking. When I apply WD-40 to the throttle cable and work it in, it shifts normally for a while then it is back to shifting hard. I only drive it on the weekends, so it sits outside the rest of the week. It shifts fine at any time from 2nd up to 4th gear.