Low-power radio sources dominate the radio sky .
They tend to be small in size and core-dominated , but the origin of their properties and the evolution of their radio plasma are not well constrained .
Interestingly , there is mounting evidence that low-power radio sources can significantly impact their surrounding gaseous medium and , therefore , may be more relevant for galaxy evolution than previously thought .
In this paper , we present low radio frequency observations obtained with LOFAR at 147 MHz of the radio source hosted by NGC 3998 .
This is a rare example of a low-power source which is extremely core-dominated , but which has two large-scale lobes of low surface brightness .
We combine the new 147 MHz image with available 1400 MHz data to derive the spectral index over the source .
Despite the low surface brightness , reminiscent of remnant structures , the lobes show an optically thin synchrotron spectral index ( \sim 0.6 ) .
We interpret this as being due to rapid decollimation of the jets close to the core , to high turbulence of the plasma flow , and entrainment of thermal gas .
This could be the result of intermittent activity of the central AGN , or , more likely , temporary disruption of the jet due to the interaction of the jet with the rich circumnuclear ISM .
Both would result in sputtering energy injection from the core which would keep the lobes fed , albeit at a low rate .
We discuss these results in connection with the properties of low-power radio sources in general .
Our findings show that amorphous , low surface brightness lobes should not be interpreted , by default , as remnant structures .
Large , deep surveys ( in particular the LOFAR 150 MHz LoTSS and the recently started 1400 MHz Apertif survey ) will identify a growing number of objects similar to NGC 3998 where these ideas can be further tested .