Goofy Shake
 

I can't figure out what's causing my car to shake...it's not really a shake though.

It's kind of a hard thump going back and forth from front to back. There is no side to side movement or vibrating...just front and back.

It almost feels like somebody pumping the brakes. it doesn't do it all the time but will go for 10-30 seconds every once and a while.

when does it do it? on the gas, on the brakes, in the air, under the water?

Re: Goofy Shake
 

Sounds like you might have one spark plug that's not firing all the time. Have you performed any maintenance on this car recently? If not, I would do your basic "tune-up". Replace the Spark Plugs(Not Bosch, stick with OE Delco), Wires, Air and Gas Filters, PCV, and O2 Sensor(again, NOT Bosch).

no, I haven't done anything to the car since I got it a year ago other than replace a bad TB gasket (I think that's what the problem was)

On the highway it would start doing this @ the crest of a hill going up. But in town it does it just randomly it seems. It stops when I let off the gas.

Maybe my transmission isn't engaging fully and slips?

I would do the "tune-up" thing first BEFORE suspecting the transmission. A LOT of people here simply replaced the ignition wires and the symptom you described went away. Also, it would be a good idea to drop the pan on the transmission and replace the xmission filter and oil. The procedure for this is in the Tech Info section of this board.