93 mustang 4cyl. , starts fine and then about 20 seconds later starts sputtering and backfiring for about a minute. After that it runs fine unless I shut it off and leave it for a couple hours, then the process starts all over again. I had it scanned and no error codes were evident. I replaced the plugs and wires but no improvement. The mechanic told me that the aftermarket plugs and wires were not as good as the motorcraft parts, also said that 4 of the plugs were "cold" and 4 were "hot". When I bought the plugs all 8 were the same and the parts store was unaware of the hot/cold plug scenario.

Any help would be appreciated.

Maybe the mechanic meant that one set of plugs was meant to be a higher heat range than the other set, and having all the plugs the same heat range means one set is mismatched. Check with a Ford dealer to see if a different part number is used for each set. I work at a Ford dealer, but I don't buy into all the crap about buying only OEM parts. Sometimes they're better, sometimes not. Sometimes they are the same as aftermarket in a different box. Anyway, you need to find out which cylinder is misfiring, then find out why. It could be anything from a vacuum leak to oil fouling, or anything in between, like a damaged spark plug/wire. The time delay seems to indicate that the rough running occurs when the PCM goes into closed loop, causing a lean skip. As the engine warms up and parts expand, the vacuum leak seals up enough to end the lean misfire. Try spraying some carb cleaner around the intake manifold gasket when it runs rough. If it smooths out, the intake is leaking. Narrow it down to the offending cylinder. If you get no joy, check into other possibilities, such as plug fouling, damaged ignition parts, carboned up combustion chambers/intake/exhaust ports, etc. etc.