That depends on your "aftermarket" gauges. If they need 12v to run on, then they only need to be wired to a switched 12v source and have a reliable ground. If they use 5V, they need to be connected to the original IVR or have their own 5V regulator. Since you've removed the IVR and the gauges appear to work some of the time, that doesn't seem to be the problem. In either case, a volt meter will tell you plenty.
There should be no voltage reading on the low or ground side of the gauge. If you find a voltage, the ground is the problem. There should be at least 12v on the hot side with the car off, over 13 when it's running, and it should be stable. If that voltage is drifting, check the wiring from the gauges back to your fuse block and check the voltage at that point as well.
For a suspect ground, use a heavy clip lead and run a redundant ground from the car's body to the ground side of the gauges. Do the same for feeding 12v into the cluster from a known good 12v source.
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now sandblasted and DP40'd since this pic was taken.
a two legged hole shot, even with a munged up carb and snow tires...