My car is lagging when I accelerate?

I have a 99minute Nissan sentry gxe ... I need help yet again... my car is having a very hard time accelerating.... it was doing this before until it eventually broke down and i had it towed to a shop where they said it was the distributor. I had them putin a used one... now a few weeks later im having the same problem. It willis be fine for aa few minutes after i start driving. When i am stopped the tachometer needle will dip below 1000 rpms...when I push on the gas pedal there is a big lag before I accelerate. The tachometer will move up to 3500rpms but I wont get any speed for a bit. Other times the tachometer will just stay down or move up and down slightly... I don't really know how to explain it... any help would be great. Is this sounding like a big fix? I already paid the shop $86 for them to just diagnose it wrong...