1995 Chevy Lumina Acceleration

Hi, I bought my 95' chevy lumina and it ran fine for the first few months, but now there is something wronge with it. There is no perticular temp. That is acts up, but its like a mind of its own. When I stop sometimes its doesn't have any power. I crush it and it runs 2-5 mph. I have to put it in 1st to get it to get it to go at a decent speed through the stops. I've noticed before it happens the spedomiter goes from regular speed to to 0 than after a few seconds it goes back to normal speed. I've changed plugs and wires, fuel filter, checked the pump, cut off the cat, and it seems to be still doing it. I've had the trany check out and they said it was in great condition, I also took it to a shop and they said that nothing was wronge with it. I at a total loss. Can you Help, please!