I had a similar problem in my '82 300D. The a/c would work fine for a few minutes, then the compressor would cut off causing the fan to blow ambient air. The problem turned out to be a malfunctioning low temperature cutout switch. The switch was intended to prevent the system from freezing, but it was tripping at a much-too-high temp. (The '98 may or may not have such a switch.)
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