Had same problem on son's 99 SR5. We changed all plugs, wires, and even coils from my 99, but kept coming back. Changed the injector and fixed it. Still going strong 4 years later.

Did you use denso injector #23250-62030? And are all 3.4 injectors the same? This is a '99 as well.

Used injector from dealer. Not sure of number.