robert schlatte

On my 1980 Schwinn Voyageur I recently converted a 6 speed freewheel hub to a 8, 9, 10 speed Shimano Tiagra freehub. I am running a SRAM PG-850 cassette with the following size sprockets: 11, 13, 15, 17, 20, 23, 26, 30. Everything runs pretty smoothly except when I'm on the 17T sprocket and either my big (52T) or small (39T) front chainring, it feels gravelly like it is rubbing against an adjacent sprocket. When I am on the big front ring the chain line looks like it runs very straight on to the 17T sprocket. The 13 and 15 sprockets have a little gravelliness too but not as pronounced. I replaced the 8 speed chain with a 9 speed one but that did not help. I am a little stumped. i think I will try a 7 speed cassette with greater spacing between sprockets. Anyone have experience with anything like this? Any ideas? Thanks.