You say '3 different thermostats.'
I'm assuming you are talking about the temperature probe? (that's what I call it)
If so, that was my first guess, but since you've tried 3 of them, likely not the problem. I'm chasing a similar issue in my 74 Bonne right now, so I feel your pain.
You mention the gauge spikes when cranking, so electrons are moving.
Maybe PM Pete Serio here on the forums. He seems to have a lot of great knowledge about the Rally gauges.
Hope that points you in the right direction.
__________________
Eric "Todd" Mitten
'74 Bonneville 4dr Sedan (455/TH400/2.93 open)
'72 LeMans GT (455/M-13/3.23 [8.5"] posi)
'71 GTO Hardtop (400/TH400/3.07 12 bolt posi)
‘71 GTO Convertible (455HO/TH400/3.23 posi)
'67 GTO Coupe (455/ST-10/2.93 posi)
'67 Tempest Wagon (428/TH400/2.56 posi)
Deuteronomy 8:3
