engine runs rough, then gets better

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

A '93 Mustang 4 Cyl with 8 Plugs.....mm hmm.....Belongs in the Mustang forum and its gots a V8 (8 cyl), but...check the gaps on your plugs make sure they correct for your year make model and edition car. After that I'd look for some type of vacuum leak, while its running rough, something that might be a leak, but then when it warms up it expands and closes itself off, maybe your plugs wires or distributor even.....those are my thoughts.

Sounds like you need a new O2 sesnor. As soon as it's in closed loop mode (warm) it's getting the wrong signal from the O2 and try's to over correct it.