Thanks a lot for the ir-ctl output!
It looks similar to a rc-6 signal, but not quite. The pulse/space lengths should be 222-666 or 666-1110 (except for the first one and another pulse/space mark at the beginning which are longer).
Your output shows some very small values of about 100 or less, so the signal can't be decoded properly.
Which rev of the atric IR-Einschalter are you using? The manual of rev5 mentions changes that resolved issues with some com ports. A mismatch in signal levels would explain these length deviations.
There were a couple of posts on the atric forum mentioning issues with unclean/noisy power supplies (and having to add resistors and/or filtering caps or a newer/fixed IR receiver+cable to cure them). This could also explain the issues, most IR receivers are rather sensitive to noise on the power supply.
On the system/software side the only thing the only thing that could cause these timing variations is IRQ latency. Check /proc/interrupts and make sure serial_ir is the only device using IRQ4, if some other device is using this IRQ as well go into the BIOS setup and move that device to it's own interrupt.
so long,
Hias