I have a problem with my 99 Jeep Cherokee SE(4.0). Each time I start the engine and go for about 1/4 of a mile, the engine starts to sputter and lose power, but does not stall. After about 3 to 4 seconds, the problem goes away until the next time I start up the engine again. I have replaced the crank position sensor and the ECU to no avail. Check codes and there are no codes, no check engine light. Also replaced spark plugs and distributor rotor and cap. Any suggestions?