Problems with a New Beetle - sluggish acceleration and stalling

My 18 year old daughter has a 98 New Beetle with a recurring problem. It starts with sluggish acceleration, and increased gas consumptions and then move on to intermittent stalling out. If you wait 5-15 minutes it starts again. We took it in and they had problems diagnosing the problem because it wasn’t throwing an error code. However, they finally got it running but I never got a a definite diagnosis on what was wrong. They told her to stop buying gas at the cheap stations, which she did. Well six months later it’s starting to do the same thing, the acceleration is gotten so sluggish up hills that people are passing her. Since the only VW repair place around here can’t get it looked at until after Thanskgiving, and I don’t want her driving around in a car that might start stalling out, I’m loaning her my car. So I’m anxious to find a solution. Any suggestions on what the underlying problem is here?