I have a 03 Navigator that just had the engine replaced, now it shutsoffrandomly

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Ever since I had the engine replaced, since I got it back it shuts off randomly when on. It starts immediately when I turn the key to start it. Another mechanic who is not certified is telling me that I need a whole new computer system. The car will turn off when stopped at a red light, idling in park, or when im driving down the freeway. Is there any knowledge as to why this could be happening. Please help because I do not want to scrap my truck with a new engine replacement that i just paid for. The engine put in it has 80,000 miles on it