I would start by getting the battery it is powered from load tested to make sure it is still good. We’ve seen this on other things caused by lower power issues. The voltage may look right but there is not enough current flow to keep the 987 unit running correctly. I would next try checking all of the electrical connections between the unit and the battery. No loose or corroded connections – even in the fuse holder and the fuse. If you have not done so in the past you may want to run a new set of 14 AWG multi-stranded copper wires from the unit to the battery (with a new 3-amp blade type in-line fuse). Solder and seal all connections.