1997 Jeep Wrangler loss of power, stumble

My 1997 Jeep Wrangler will occasionally loose power and stumble. This happens at any and all operating temperatures. Sometimes I can drive 100 miles between occurances. The engine never cuts off completely. I have 50# fuel pressure, NO check engine lite or trouble codes. When this happens the engine vacuum drops to 4 to 8" and it triggers the map sensor to richen the mixture and increase the injector dwell time and it run rich. This condition usually lasts approximately 2 to 3 minutes and goes away. I can watch a vacuum gauge connected to the intake and actually see the vacuum drop and them come back up. Where can an internal vacuum leak be?