[66sprint]

Did you replace the 3 brake hoses?
I had a similar issue on an older car and found one of the front hoses had spilt the inner liner allowing fluid to pass into the outer part of the hose.
It never leaked but you had to push a bit on the pedal before it built up enough pressure to activate the cylinders making them suddenly grab.
If you haven’t put on new hoses, try feeling the old hoses for soft spots.