engine shuts off

Have 89 ford ranger v6 manual trans.engine will run for 20 mins or so and then just shut off. After cool down will start up and do same thing.
Change ingition module (tfi) same thing happens ,what else can I check.

what motor do you have? If it is the 2.9 it is probable that your heads are cracked. sporadic shut off was the first sign before my heads went to the great graveyard in the sky. I suggest you do a compression test. how does it idle/run before it kills?