1998 Ford Explorer Higher RPM when I start my car

For the past 3-4 weeks when I start my car in the mornings I noticed that the engine starts real slow for a couple of seconds then it seems to finally go and the RPM range from 500-2000 while I hear a revving sound. This will usually last up to 10 seconds or so. Once this passes my car seems to run just fine but everytime I hear it turning I feel like it's just going to die on me. I took it into a mechanic and they recommended a new starter for my car. I went for a second opinion and was told that it was the Idle Speed Control Valve, which he proceeded to adjust but the problem is still there. Any advice would be appreciated. Note: I've just had a tune up, oil change, tire rotation and replaced the air filter this past week as well.