I have 98 STS with 85K on the clock and since it's gotten cold here in Northeast Wisconsin, I have developed a misfire when the engine is started cold. The engine idle is somewhat erratic like it is trying to fine the correct RPM, but as it runs for a brief moment, the idle lowers and it runs rough. Once it warms up, it runs great. I was thinking maybe it is a vacuum leak, but I thought I would ask you guys first. It sets a p0300 code, but it won't tell which cylinder is having the problem. I just replaced the plugs, but it didn't help at all. Any help is much appreciated.