Richb, have you been shooting offhand or off a rest? Have you recovered any of the patches and examined for signs that the patch is getting cut or burned through during the loading/firing process? Are you cleaning between shots? The only thing that caught my eye in reading your description was use of Teflon patching - I didn't know Teflon patching was even still available.
The things that come to mind to try after you've tried the previously made suggestions are:
(1) get a more experienced BP shooter (preferably one who shoots a 45 so you can try a sample of his patch and lube) to go to the range with you. Let him shoot your gun with your loads and see if the good and bad groupings stay with the gun when he shoots. If 100 yard performance is noticeably better when he shoots, you know you just have to work on technique a bit. Recover fired patches and check for any sign of damage. Ideally, the patch ought to look sound enough to be able to use again.
(2) Find a good article on 'working up a load'. Try some different ball diameter, patch thickness, patch lube, and powder charge combinations. If nothing else, lay hands on some pillow ticking in the 0.018 to 0.020 thickness range and lube it with Hoppes 9+, wonderlube, spit, or Mr. Flintlock and see if that makes things better or worse.
(3) Until you find a load that groups to your satisfaction at 100 yards, shoot off a rest.
Good luck, SCL
P.S. It occurred to me that if you're old enough to have had the Hawken in a closet for 20 years, you're probably old enough for your vision to be changing some. When you put the Rice Barrel into the TC stock, what barrel length and style of front/rear sights did you choose, and how far forward of the breech did you put the rear sight? I would be very surprised if the issue is a defective barrel and would be looking at a load issue (like maybe a patch failing) or a sighting issue (can't see the distant target well enough to aim at the same spot on it, or having trouble keeping the same rear/front sight focus for the distant ranges).