Hi

My truck runs perfectly fine when I first start it and even sometimes after I've stopped it then started it again. (For example if I stop to go into the store). But sometimes after a while when I start it up again it revs up to about 1000 rpm then drops to about 500 rpm. After I've driven it for a while after that happens it goes back to around 850 rpm like it should.

 

Check all the vacuum hoses and intake tube for cracks/leaks. Vacuum leaks can cause the idle go weird.

 

Also check that your oil dipstick is securely inserted & the oil fill cap is tight.