Print freeze after resend request

Hi all,

Our print freezes each time at exactly the same line of gcode after a request resend.

The setup:
- Mendel v2
- Gen3 with Pololu A4988 extruder driver
- FiveD firmware (2010 08 06)
- Slic3r 0.7.1
- RepG 34

We know both Gen3 and FiveD are depreciated, but for us this combination works fine most of the time with surprising good print quality.
So we really want to fix this annoying problem. (when we have a Solid State Relay for the HBP, we'll give Teacup fw a new try)

First try:

[21:31:19] Sending: N4701 G1 X102.103 Y91.341 E2.14056 *75
[21:31:19] << rs 4701 serial error: checksum mismatch.  remote (7) not equal to local (75), line received: n4701 g1 x102.103 y91.341 e2.14056 *7
[21:31:19] Received resend request for line 4701
[21:31:19] Searching: N4701 G1 X102.103 Y91.341 E2.14056 *75
[21:31:19] Resending: "N4701 G1 X102.103 Y91.341 E2.14056 *75". Resends in 1 of 0 lines.
[21:31:19] Sending: N4701 G1 X102.103 Y91.341 E2.14056 *75
[21:31:19] << ok
[21:31:19] FW Accepted: ;resend-ok

Second try:

[22:34:03] Sending: N5640 G1 X102.103 Y91.341 E2.14056 *78
[22:34:03] << ok
[22:34:03] FW Accepted: N5640 G1 X102.103 Y91.341 E2.14056 *78
[22:34:03] Sending: N5641 G1 X102.439 Y90.558 E2.16640 *79
[22:34:03] << rs 5641 serial error: checksum mismatch.  remote (79) not equal to local (121), line received: n541 g1 x102.439 y90.558 e2.16640 *79
[22:34:03] Received resend request for line 5641
[22:34:03] Searching: N5641 G1 X102.439 Y90.558 E2.16640 *79
[22:34:03] Resending: "N5641 G1 X102.439 Y90.558 E2.16640 *79". Resends in 1 of 0 lines.
[22:34:03] Sending: N5641 G1 X102.439 Y90.558 E2.16640 *79
[22:34:03] << ok
[22:34:03] FW Accepted: ;resend-ok

I've been monitorring the communication from RepG and up to this point there were no 'Resend Requests'
The Electronics accept the line after the Resend but either RepG doesn't send the next line or the firmware is stuck afterwards.

Does anyone have a clue what could be the problem or how I could start debugging?

Thanks on advance.

Grtz, Tim