MAF problems

1999 3.8L mustang 140,000 miles. I bought this car used and whoever had it before me put an aftermarket filter on it. Not an entire intake, but removed the stock air box and put an open filter in its place. I recently started having problems, detonations, erradic idle, hesitation, lower gas mileage. I finally took it in becuase it died while driving. The mechanic said it was the mass air flow sensor, so I went to autozone and replaced it. The idle and misses are better but the car died on me again, and 3 more times on the way back to the mechanic. He couldnt reproduce the dying, but suggested to replace the open filter with the stock air box. Since I never had it, and had no luck at the junkyards I went to a tuner shop. The mechanic there told me that it should work and the problem was the aftermarket MAF and to get one directly from ford. So my question is, should I keep looking for a stock airbox, get a new MAF from ford, or buy an aftermarket intake designed for my car.