From what George said I might pay to check your old waterpump. If it did not make the bypass system functional then that is why the "PI" thermostat worked.
When you changed to the Edelbrock pump you made the bypass system functional and now require the thermostat to meet that need.
My car had a Weiand pump which does not have the bypass provision and it used a stat like your "PI" stat.
Its worth investigating to know for sure.
I would question Georges advice also, ever since the hot chick avatars and alcohol product placement in his pictures dissappeared I have been worried about him....
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