connection fails randomly

Hi!

I'm using Marlin on a Gen7 1.5 board with repetier host. I have a serious issue which happens with different PCs and different firmwares, even different USB cables. The connection suddenly fails after a certain amount of time. I'd say it is always about 1-10min after the start of a print. It doesn't happen when I don't run G-Code, or not that soon at least. This leads me to the conclusion, that it has something to do with the actual data transmission, meaning: not after a certain amount of time, but after a certain amount of transmissions.
I understand that the host sends commands and the printer answers "ok". Here's the log when it occurs:

...
15:58:11.864 : N592 G1 X108.73 Y106.734 E0.00709 *120
15:58:11.880 : ok
15:58:14.890 : N593 M105 *8
15:58:14.922 : N594 G1 X93.266 Y91.27 E0.27157 *121
15:58:14.937 : ok
15:58:17.948 : N595 M105 *14
15:58:17.979 : N596 G1 X93.837 Y91.27 E0.00709 *125
15:58:21.006 : N597 M105 *12
15:58:24.063 : N598 M105 *3
15:58:24.094 : N599 G1 X108.73 Y106.163 E0.26154 *125
15:58:24.094 : o

it is always only the "o" left. Then, nothing happens. The printer continues to keep the temperatures and just "waits". Also the PC and host continue to work. After I restart the printer (and dis- and reconnect the USB cable) I can connect to printer. I don't have to restart repetier. I sometimes observe similar things when not executing G-Code. I reconnected the printer and this was the log (without commands)

16:17:49.512 : No start signal detected - forcing start
16:17:49.528 : ok
16:17:49.528 : ok
16:17:49.543 : FIRMWARE_NAME:Marlin V1; Sprinter/grbl mashup for gen6 FIRMWARE_URL:[github.com] PROTOCOL_VERSION:1.0 MACHINE_TYPE:Mendel EXTRUDER_COUNT:1 UUID:00000000-0000-0000-0000-000000000000
16:17:49.543 : ok
16:17:49.559 : X:0.00 Y:0.00 Z:50.00 E:0.00 Count X: 0.00 Y:0.00 Z:50.00
16:17:49.559 : ok
16:17:49.559 : ok
16:17:49.559 : echo:Active Extruder: 0
16:17:49.559 : k
16:17:49.559 : ok
16:17:49.559 : ok
16:17:49.559 : ok
16:17:49.559 : ok
16:17:49.559 : echo:Active Extruder: 0
16:17:49.559 : ok
16:17:52.928 : ok

You can see that there is also an error with the "k".


I also observed that before this error, the printer starts to make breaks between the moves. It executes it, waits 2s, continues with the next move, and so on.

Any hints?
Thanks!