My '98 Blazer (W code) with 86,000 miles runs fine when fully warmed up, but very rough when cold. This happens even in warm weather until the indicated temperature is about 'normal' (about 190F). The engine always starts right up and idles well, but is very rough on acceleration until warm. The Service Engine Soon light usually comes on and eventually goes out after a period of running when fully warmed up. I've had the codes scanned twice and both have been inconclusive; one suggested the fuel pressure regulator and another a #4 misfire. The engine seems to take a long while to reach full warm condition, but when it does the misfire clears up. Any ideas?