what would cause engine to lack power and buck?

If it is the 4cylinder confirm ignition timing bridging T1 and E1 in the diag connector box, check also that the timing chain and guides/tensioner are OK. On V6 3VZFE engine reconfirm valve timing, make sure ignition coil is not"breaking down" with engine running spray it and the ignition system with water from a spray bottle, if insulation is good the idle should stay the same and the engine not cut out. Check for vacuum leaks and verify fuel pressure and delivery volume. There is a company that independent auto repair shops get their information from, this same information is available inexpensively for people that work on their own cars.. The information is year make and model specific, covering repair procedures, torque specifications, fluid capacities and specifications, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online