Need help with vibration in new motor!! (5th gear shimmies) PLEASE HELP!!

[Gearhead999]

If the car runs good cold then starts the "shake" as it warms up. I would lean toward the ECT might be bad. The ECT gives the ECM input as to the engine temp. so it can richen up when the engine is cold, like a choke on a carbed engine. As the engine warms up it leans the mixture down.

Get a scanner on it and besides the codes you can monitor what the sensors are sending. It will surely "shake" if it is lean. What do the plugs look like??