Hey guys, I am riding a recently built up late 80's Vitus 979. It is originally a 6 spd with Shimano 600 components, but I have recently put on a new set of Bontrager Race wheels that I assumed couldn't take the old 6 spd freewheel (right?). So, I now have an 8 spd cassette but the same old shimano 600 shifters and derailleurs. I am running the shifters in friction mode and I am able to switch from gear to gear fine. BUT, when I get out of the saddle and really pump hard or climb a hill the bike will shift into a higher gear, which can be problematic obviously when climbing a hill.
Is this simply because the Vitus is so 'flexy' and puts tension on the cables when I really push it? Or is it my 8 spd cassette not interacting well with my 6 spd components? how can I alleviate this?