Volvo Repair: 91 740T A/C Fades out on Acceleration, vacuum leak, vacuum lines

Question
1991 Volvo 740 Turbo, auto trans, California car, 165,000 miles: The Air Conditioning fan (also in fan mode only, too), "fades out" - that is, gradually stops blowing during acceleration.  As soon as I back off acceleration or settle on a steady speed, the fan "fades back on".  The A/C blows cold and has never had any problems before.

Thanks.

Answer
there is some type of a vacuum leak in the air conditioning ventilation system.I would check all vacuum lines in the engine area.if they are all ok it could be a bad vacuum resevoir which is under the engine or behind the front spoiler or even a bad check valve or bad vacuum diaphragm.