Have a Shimano 600 SIS/Index 7 speed downtube shifter on a late 80s Cannondale road bike with a Sante rear derailleur RD-5000. The lever stays in the SIS position fine, but when I switch to friction and move the lever at all, it goes back to SIS. Any thoughts on how to fix this. I replaced the cable and the shifter screw is tight. Any thoughts appreciated.