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For the HEI mod - one thing that was left unmentioned was that you have to disable the stock ignition module. If your stock module is bad and is shorting the trigger signal - the external HEI module will not work no matter how better/powerful it is.
You can disable the stock module in two ways -
-Remove the module from inside the disty
-Cut the connections that go to the module (I cut the orange wire and the adjacent wire - it is important to cut both because the other wire is the stock module ground - if you left it uncut the stock module can still short out the internal coil)

I gutted the disty already ..no oem module , no oem coil, no ground for oem coil/ignitor.

Can you post pics of the gutted disty and the hei wiring just so people can spot anything obvious that was overlooked before. There might be a metal spot that has very little silicone cover and its arcing when warm - maybe?

Have you replaced the coolant thermo sensor?

Also try running the motor with the o2 sensors unplugged - it might throw a code and put the ecu in limp mode (which can be fixed by a reset)- but atleast you can take off o2 sensor from the list.

Another thing that affects the engine when warm is the EGR system - Have you blocked the vaccum hose to EGR by mistake?

Have you tried pulling codes?

Those are the things i could think of right now - Since you mentioned the tach going to zero suddenly - IMHO it's most likely a disty/coil problem - I would probably get a junkyard disty just to eliminate any doubts about the disty setup.