disagreement between theoretical calculation and measurement of upper cutoff frequency of a circuit based on RG58 coaxial cable
You need to model your lumped line in much smaller quantities like "per metre". At the moment you are modeling it as "per 100 metres" and that will give you severe problems above a fairly low frequency.

Consider this 25 uH and 10 nF have a resonant frequency of: -

$$\dfrac{1}{2\pi\sqrt{LC}} = 318 \text{ kHz}$$

And clearly that is the problem with using a lump of line equal to 100 metres. If you were only interested in frequencies of a few tens of kHz then a 100 metre model would be fine. So, take the maximum frequency you are interested in and convert to a wavelength hence, 5 MHz has a wavelength of 60 metres. Now make sure that the lumped model length is no longer than one twentieth this distance i.e. 3 metres.