We've covered size, and there are only two other things left that can make a difference of that magnitude, lube and alloy. If you are using WW, WW+2% tin, or 50/50 WW/lead, we can pretty well rule out the alloy. Pure lead or linotype could give problems, but I doubt you are using either of those.
I have no experience with the Star lube or the Star sizer, but I am deeply suspicious of all hard lubes. Had more than my fair share of troubles with them. If you can get some bullets sized without lubing them, try a light coat of liquid Alox or even Johnson's paste wax.
I get 50 yard groups in the 1" - 1.5" range with Lee DEWC TLs from a Handi. There just ain't no sense in cast WCs shooting that badly for you. I can see groups opening from 1.25" to 1.75" or even 2.00", but opening to 5" says that there is something major wrong.