Pfaff 335 – Hook going backward when lifting presser foot (missing stitch / broken thread)

[OlbT]

Hi everyone,

I'm having a frustrating issue on my Pfaff 335 and I can't figure out the root cause.

The problem: When I stop with the needle down, everything seems fine. But as soon as I lift the presser foot, the entire machine jumps backward by about 10° — you can feel it directly in the hand wheel, so it's not just the hook itself.

Because of this backward movement, the hook doesn't catch the thread loop correctly on the next stitch, which results in:

• Missing stitches
• Broken thread

What I've already noticed:

It only happens when lifting the presser foot (not during normal sewing)
The recoil is felt throughout the whole machine, not just in the hook area. (the hand wheel is also going backward) 
It's consistent and reproducible
When turning the hand wheel manually through a full cycle, the resistance is very uneven: there are positions where it feels very heavy and hard to turn, and others where it rotates much more freely. I'm not sure if this is normal for this machine or a sign of something wrong.

Machine specs:

• Model: Pfaff 335-G-6/01 BLN
• Control box: SV71-S
• Motor : SM71S-6535-IM1A 650W

I attempted to adjust the servo motor configuration to see if the needle-down stop position or brake behavior could be tuned to prevent the recoil, but I was unable to achieve any improvement.
Has anyone experienced this before or have any insight into what's causing it and how to fix it?

Thanks in advance!