i have a 98 5.2 with 140000 on it. when i start it up whether cold or not it runs good for about 10 seconds then the rpms start to drop and they go as low as 300 rpms and nearly stalls out. the exhaust smells so strong that it makes your eyes water. after about 2 minutes of this it fixes itself by reving up and then then runs like a top. also sometimes when i will accelerate it will nearly stall and i will let my foot of the gas for a second then accelerate again and it will go perfect. i am completely stumped don't know even where to start.

my check engine light is not on.
i do not have a cat on my truck.

any help would be greatly appreciated