The nut under the handle (packing gland)is a cronic problem on most stem type of valves like a gate and globe pattern.
Even under the best conditions the packing does loosen and thus leaks.
You could of course try to get a professional Plumber to install a ball valve (Bronze) or possibly open your leaking valve fully THEN back off a 1/4 turn so it doesnt freeze in the open position and then VERY GENTLY tighten the bonet nut again just until the leak stops.
When you need to close this valve for a system shut down you can back off this nut then close the valve so your not putting undo stress on the cheap plastic parts then tighten the nut if it leaks until you need to open the main again.
Sometimes all you need to do is either fully open or close a valve remove the handle remove the nut remove the old packing hope it isnt graphite as it can score a stem then repack the stem and retaining nut with 1/8 diameter Teflon PACKING not the tape but packing.
When repacking do NOT use it in one continuous strand but cut several short pieces like I did for very high pressure steam applications (1,200-1,500 PSI)and face each piece 180 degrees apart.
Simple basic plummmin stuff.