  We carry out experiments on single-molecule junctions at low temperatures,
using the mechanically controlled break junction technique. Analyzing the
results received with more than ten different molecules the nature of the first
peak in the differential conductance spectra is elucidated. We observe an
electronic transition with a vibronic fine structure, which is most frequently
smeared out and forms a broad peak. In the usual parameter range we find strong
indications that additionally fluctuations become active even at low
temperatures. We conclude that the electrical field feeds instabilities, which
are triggered by the onset of current. This is underscored by noise
measurements that show strong anomalies at the onset of charge transport.
