Finally found the right settings for my TL-WR1043ND (DD-WRT).

Wireless Network Mode: N-Only
Channel Width: Turbo (40 MHz)
Wireless Channel: 13 - 2472 MHz (but any other channel should do, too)
Control Channel: lower
TX Power: 50 dBm
Short Preamble: disable
Sensitivity Range (ACK Timing): 0

So far, no more dropouts. I think the key is to use a specific network mode and a specific channel width and avoiding mixed (or dynamic) modes.

Regards,

J4ZZ