Early dart electrical gremlin???

Offhand, I'd say you have a grounding problem with your dash cluster

WHILE YOU HAVE IT OUT, and that's what you probably need to do, check a few things carefully:

(Not that hard to remove. Drop the column, remove the cluster screws, reach up and unscrew the speedo cable. Fold the cluster "down and forward" and it should come out then unhook the board connectors CAREFULLY)

Check the connectors from the harness to the cluster PC board. They get loose, corroded, etc. Solder them to the board, and clean the connectors

Remove the attaching screws from the PC boards / casting and clean the board where the screws contact, tighten/ loosen the screws to scrub them clean, better yet find new screws and use INSIDE STAR washers under the screws

Loosen/ tighten the nuts on the temp/ fuel gauge

Solder the voltage limiter contacts (the brass springy things) from the brass to the board

Make up a ground pigtail, put it under one of the screws on the cluster ground, and hook it to the column support behind the cluster