I have a Shimano Tiagra derailer on my 9 spd cassette and it's been behaving really strangely.
On the road, when I attempt to shift to bigger cogs, the chain will struggle to move up, sits on the bigger cog for a moment, and then jumps all the way back down to the smallest cog (sometimes just 2nd smallest).
There is definitely a feeling of difficulty of movement coming through the lever when I try to shift.
However I have it in a stand right now and it shifts up and down, albeit awkwardly and noisily. I'm not sure how to make the proper adjustments if the issue isn't happening while the bike is stationary. It's working somewhat fine now, but once I get on the road, it's unhappy again.
I can identify the different parts of my derailer and have a feeling it's a problem with the cable tension, but I'm clueless as to how to address the issue.
Thanks, any advice will be a big help!