1987 Ford Mustang rough idle

Engine Performance problem
1987 Ford Mustang V8 Two Wheel Drive Manual

ROUGH IDLE.
we rebuilt the engine 8000 klms ago and i stored the car last winter. i have had some small idle problems since the rebuild. the build was done by ford technicians with parts i brought to them. the engine block was redone at a shop nearby.
first we put speed pro pistons .30 over the cam is a mild racing cam but i am not sure of the lobe.
everything is new with a 75 mm throttle body.

when i start the car it idles fine after 3 to 5 minutes
after warming up it starts to rough idle jumping from 1200 rpm to about 700 or 800 and it does this usually for 5 to fifteen minutes before it stalls
when i am driving there is no problem but when i press in the clutch sometimes she stalls. sometimes i can drive 50 to sixty klms come to a stop and it doesn't do it. but most times it does. i would like to solve this problem because every thing is power, brakes and steering and it's getting frustrating

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