1. It amazes me that your 3.4 has lasted that long. You must be doing something right.
2. You problem sounds typical of a vacuum leak, the engine surges while the control system tries to compensate. Try spraying water, ether, or wd-40, whatever, around the throttle body, egr, iac, and intake seams. If it hits the leak, the motor will stabilize momentarily as the fluid blocks the air leak. Then you have to pinpoint where it was drawn in from.