Siegfried

I have my Skywatcher AZ-EQ6GT mount now a little over a year.
No night I manage to follow an object well. Sometimes after a few minutes or sometimes after an hour, the mount jumps back and forth.
My setup is an AZ EQ6 GT with GSO RC-10, controlled via USB cable Hitech.
Everything is controlled via EQMod (latest version).
The mount itself is perfectly polar aligned.
I've tried various USB cables (HiTech, Pierre Astro and even a Chinese cable)
The tracking camera is a ZWOptical 120MC. The tracking telescope is a ED-80 (480 mm). Guiding software is Meta Guide or PHD Guiding. To find objects, I use Stellarium which guides the mount by EQMod and Meta Guide.
This works but after a few minutes my mount jumps again back and forth.

After more than a year misery I bought me last week the Lacerta MGEN II and controlled the mount via the supplied ST4 cable. Searching for objects and slewing to them is still made through Stellarium and EQ Mod. After a few minutes guiding the montage starts again to get cures.
In desperation I removed all USB cables with only a connected Synscan. Guiding works fine now, even after one and a half hours.

Some examples:
NGC 7023 through EQMod and Lacerta, guided for one and a half hours with EQMod.
http://geryl.be/astro/irisnevel.mp4

Capella guided only with SynScan for 1.5 hours.
http://geryl.be/astro/capella.mp4

PS: The picture quality is not good because there was some wind and veil clouds.

Anyone whith an idea what I am doing wrong?