I started having a weird problem with my 2006 Fiesta ST when cold starting the engine, it starts up with no problem but then the revs drop low like the engine is going to die and then surge back up and it does it about two times. But after that engine idles normal and when driving I have no problem with idling. I have already cleaned the throttle body and unplugged battery which solved problem for about 3 days and then problem returned. I also have pretty new spark plugs, battery, fuel filter and air filter. Any idea what is causing the surge when cold staring engine?

The only time I had uneven idling issues was when coasting in neutral, the revs would hunt.

And vacuum leaks but that typically caused rev hang. The PCV hoses can split and cause a vacuum leak but not sure if the symptoms are same as you're having.

I was going to suggest fuel filter but you've already eliminated that.

Is it running rich? ...or lean, or any telltale signs on the old plugs when you replaced them?

Likely PCV hose split.