RG-58 is just plain junk cable, and should never be used or (especially) trusted to
yield meaningful results in any measurement situation. The center conductor flops
around inside the shield at the slightest provocation (even quantum fluctuations),
leading to impedance variations from location to location as well as to phase variation
with bending. Further, RG-58 and similar single-shielded cables often have very poor
shielding integrity.
Also, with just about any cable, if it is poorly made it can exhibit so-called "structural
resonances" due to spatially-periodic variations in characteristic impedance arising
from the manufacturing process. Normally this is a problem only with cheap cables,
but in principle could occur with any. The result can be a surprisingly-sharp increase
in attenuation at and beyond the resonant frequencies, even though said frequencies
be far below the first "moding" frequency.
Dana K8YUM
