Rough Idle

[ford2774]

Hi All!
I have a 1992 T4R 4WD 3.0V6 133,350 mi
New plugs in 3/2006
In the past at a cold start the idle would go to 1500 and go down to 900-1000 when warm.
The past two weeks at a cold start the idle goes down to 500 and sputters/shakes and then slowly go to 900-1000.
It idles normal when starting when warm
I tightened the clamps on the air intakes, and it helped a little.
Due to high prices I have been using cheaper (Quick Trip) gas.
Any ideas?

Thanks!
Martin