Whoa old post...I'll chime in. I had this code p1399 problem and it was indeed a cylinder misfire. This was caused by running the car with a loose air filter for a month.
I secured the air filter and housing but idle still present.
No valve leaks yet the air sensor was thrown off.
Next, the engine was sucking in much more air than normal.
RPM at idle was above normal, constantly @ 2000k ...
Ignoring this problem (thinking it would go away), caused the distributor to blow out.
I had to replace it. I did.
The idle problem was still present..
Finally, I decided to change the spark plug wires..and the problem went away.
Change your spark plug wires..