I have a 98 TJ 4.0 Sport- 92000 miles. after about 2-4 minutes of operation, it starts to kick back throught the breather, pop and loose power for around 4-5 seconds. once it does this, it runs fine. it usually does this when you first start driving. i have replaced wires, plugs, distributor cap and buttum. plus replaced throttle positioning sensor, fuel sending unit and had it put on the diognostic machine twice. even the dealers are clueless. i see that there is a "upstream" and "downstream" O2 sensor, but what does this mean, where is it and will this help. i also just had the catillitic converter removed...same problems. what do i do now?