The bike is linked in my signature.

I have changed the wheels, that is, I had Shimano 600 - 6 speed rearwheel, and put an Ultegra 8 speed wheel, downtube shifter is in friction mode, and the rear derailer is set up to work with all 8 cogs, no problem.

I haven't changed the chain! There is space between the chain and the next cog and I cannot hear any buzz or crackling when pedalling, so it seems the previous chain works with the 8 speed cassette.

Here's the problem:

I usually use two gears for flat terrain, 42-17 and 52-17 when the cadence gets too high on 42T front. I like to "dig in" at some times, that is - to put all my weight on the pedals and use maximum power that I have in my legs to accelerate - something like sprinting. Usually when I put that kind of stress on the drive, the sort of chips and skips to the smaller rear cog next to the one I was before. It seems like I'm flexing the frame and the derailer wire in that process gets looser a bit and because of that the derailer shifts to the smaller cog. I think my frame is Columbus Aelle or Gara. I did fell, and the top and downtube were straightened, could that compromise the stiffness? I love my bike, but it's frustrating, sometimes I feel I'm riding a chewing gum, and I'm no pro by all means!

Are Columbus SL or SLX stiffer? I don't want to go for an another CV road bike frame and to sense the same flexing when pushing really hard.

Any ideas? Maybe to use a proper 8 speed chain would help?