Check your ground wires to the engine block, namely the little straps from the firewall to the valve covers. These typically get left off after the rocker cover has been off for a valve adjustment or a new gasket.
Or, you may have a direct ground to the engine from the battery (good cranking), but a poor ground to the cab from the battery (low voltage to the coil).
Check it out and let us know, we are curious
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