2003 Honda Element Engine Surging After Acceleration.

We recently bought a used Honda Element. The weather has been cold and my wife was saying the transmission was slipping. After checking the fluid it was within the acceptable level. The vehicle being warmed up I took it for a drive. Driving around our neighborhood doing stops and starts proved nothing. Taking it to the city streets still proved nothing. I then took it to the interstate brought it up to the speed limit and got off at the next exit. Still nothing. Getting off the interstate and returning to the city streets then it started "slipping". It didn't seem like the transmission but the engine. I would mash the gas and it would slightly stall and then a "rhythmed surge or knock". Driving back to my neighborhood I would deliberately mash the gas and the same thing would happen each time. Driving around my neighborhood once more the problem this time was there. Pulling into my driveway and putting it in Park I revved the engine after some acceleration the engine would do a rhythmed surge. Even after a slow acceleration the surge kept happening. After warming the vehicle up in the driveway and driving around proved nothing, but driving for a period of time brought the problem out. If you could help me I would I would appreciate it. Thank you!