Machine "stutters" (after inverting parport pin not in use)

Dear all,

I have a very strange problem here. I'm using Linuxcnc on a PC and, upon adding a single line:
setp parport.0.pin-16-out-invert 1
my Machine, which works fine otherwise starts to stutter and no longer moves smooth.
(The PIN is supposed to steer a relay and it does not matter if that relay is connected or not or if another PIN is used)
I also tried using a not hal component with similar results.

The PC, an Dell Optiplex 980 provides a good realtime performance and has been in use with Linuxcnc for a couple of years now. I did however recently switch the Machine and replaced the stepper drivers (went from DM556 to RTA drives)

Main hal file: pastebin.ubuntu.com/p/tqCrBQy4R6/
Ini file: pastebin.ubuntu.com/p/GbXpcmt7qh/

I'm kind of clueless here and any hints how to solve this would be appreciated.
BR,
Malte

Edit: I'm also willing to invest in a Mesa Card or in changing the stepper drivers (or anything else), just would like to be reasonably certain that this fixes the issue

ok I feel a bit embarrassed now.
I did another thorough examination of the stepper tuning page ignoring the settings of my previous machine. I think my PC was running at the edge and small changes caused it to tip over.
Base period = 20000
Steplen / space = 20000
Dirhold /setup = 80000

did the trick. But this did cause me to take a couple of days....