Homemade gauge test tool, C-3826

There are many threads on these issues. Some of them are

1....broken/ loose harness connector pins at the PC board. Clean them and solder them to the board traces

2...Ditto the contact fingers for the IVR (instrument voltage limiter) Solder jumpers from the contact fingers to the board traces

3...Poor connections at the gauges studs. Loosen/ tighten the nuts several times to "scrub" the connectors clean, and consider replacing the nuts

4...Flakey / bad IVR replace with a commercial solid state replacement or google search one of the many DIY circuits

5...Add a grounding pigtail to a common point on the cluster and bolt it to the dash frame/ column support

6...Don't discount poor connections in the power circuit feeding the cluster, or to bad connections leading to the senders. This includes the left kick panel connector (tail harness gas tank sender) and the bulkhead connector (temp sender and oil if used) as well as poor connections at the senders that is the wire end itself.

And then of course the gauge units themselves can be bad