We present new JVLA multi-frequency measurements of a set of stars in transition from the post-AGB to the Planetary Nebula phase monitored in the radio range over several years .
Clear variability is found for five sources .
Their light curves show increasing and decreasing patterns .
New radio observations at high angular resolution are also presented for two sources .
Among these is IRAS 18062 + 2410 , whose radio structure is compared to near-infrared images available in the literature .
With these new maps , we can estimate inner and outer radii of 0.03 ^ { \prime \prime } and 0.08 ^ { \prime \prime } for the ionised shell , an ionised mass of 3.2 \times 10 ^ { -4 } M _ { \odot } , and a density at the inner radius of 7.7 \times 10 ^ { 5 } cm ^ { -3 } , obtained by modelling the radio shell with the new morphological constraints .

The combination of multi-frequency data and , where available , spectral-index maps leads to the detection of spectral indices not due to thermal emission , contrary to what one would expect in planetary nebulae .
Our results allow us to hypothesise the existence of a link between radio variability and non-thermal emission mechanisms in the nebulae .
This link seems to hold for IRAS 22568 + 6141 and may generally hold for those nebulae where the radio flux decreases over time .