Koos,
I do not have the problem with my X5 tiller pilot. I would suspect that you have a high resistance connection between the power panel and the battery. Try measuring the voltage at the power panel when you the problem. You could run an extra temporary 14 gauge wire pair (DC return and + 12 VDC) from the battery to the power panel to see if it fixes the problem. If it does, disconnect the extra 12V wire. If the problem returns you have isolated it to the 12V supply side wiring. If the problem occurs when you take just the extra DC return wire off, you have isolated the problem to the DC return wiring.
When I installed the new power panel, I reterminated all the connections at both ends and cleaned all connections.