Hi Uwe, Alan,
Alan's hint about the IMP freqs fits neatly, including the FSK shift:
2*(147.3[+-42Hz]) - 138.83 = 155.77[+-85] ,
147.3[+-42] + 128.93 - 138.83 = 137.4 [+-42] .
In this case, you should also be able to notice DCF's tell-tale modulation
bursts on the IMPs.
I don't see any of these IMPs at my QTH. Perhaps there is some "rusty-nail"
type of nonlinearity, generating intermodulation locally. Can you DF it with
your loop? Or use a portable LW radio, tuned to Deutschlandfunk's USB above
73 es good luck
de Markus, DF6NM