Mr. Xulfiqar is on to something here, as always .
With temperature electrical resistance increases and if grounding of the cng kit is not proper it may cause such issue with CNG switch.
You can do it your self , just take a 2 feet piece of good wire, peel its ends off, connect one to -ve of battery and other to the metal body of the CNG kit by loosing a nut and twist the wire in and tight it again.
if this solves the issue then get proper connectors on the wire and fix properly.