1993 Mazda B2600 idling issue

When I start my truck after a cold night it starts right up but after about a minute of idling at 1500 rpm it revs up and down for about a good 5 minutes between 2000 and 1500 rpm then it stands at 2000 and slowly drops down to 900 which is my warm idel. I don't know what's wrong but after its done reving my car smells really hot so I don't think it's a good thing to be happening. Any thoughts would be greatly appreciated thank you :)