I have an idea. Use an MBO. It has a narrower roller and the spiral rollers are perfect for consisten tension. As my trainer once said,"It's all in the rollers".
However I realize that you cannot scrap your folder and buy an MBO, but I would do it in a heartbeat, having the funds to do so. Stahl folders are inferior in many aspects of design. However, like everything, they do have their good points though they are few and far between.....
I have ran both and an MBO is a superior design, in almost every aspect. Oh sure, you have the owners who want that "all heidelberg shop", but as an operator of 20 years, i would trade 10 Stahls for one MBO.
This is, of course, my opinion.
If your fold is inconsistent, check for marred deflectors and pans, gouges in the fold rollers and work coming off the guide. These are basics but the problem can be much more than this. If you have zero-makeready plates, they too can be inconsisten. We have an "AmeriStahl" B20 and it came from the factory with these. We bought standard swing-down deflector plates to try and solve the problem of inconsistency as well as the machine eating perforated work and it was like a new folder. Except it still feeds like crap, a trait common to these machines. I've had a Heidelberg mechanic tell me to "get a better model", but my owner seems dead-set on owning pile fed machines only and I really hate these types of feeders because we have a Komori 5 color and it curls work like I've never seen. Pile feeders exaggerate this curl making it almost impossible to run a full load without manually defeating the curl which is a real time waster. But anyway, I hope I can help. Feel free to PM me if you'd like.