Please excuse me being new to the forums and not owning a Ranger yet but I am an ASE certified technician and have factory training with Toyota. Enough of that, my dads co-worker has a 1999 Ranger FF 3.0. He had a cracked block which was replaced six years ago. Two years ago his truck started overheating. It would cause the overflow bottle to overflow and the radiator hoses very hard. No indication of water in oil and vice versa. I pressurized ths cooling system and cranked the engine over after pulling the spark plugs out. Water sprayed from cylinder 6. (right bank 1/2/3, left 4/5/6). Once I pulled the heads, cylinder 6 was cracked, and the valve seats were pounded in too deep in the heads. Apparently this is common on 3.0's, head cracking, what the machine shop told me.
Now, do your basic tests and you may need to pull your heads. If it's been done twice already then there is an underlying problem. Check deck flatness, make sure the coolant is flowing properly, thermostat is opening correctly, etc. If you need new heads, I've personally dealt with Cylinder Heads International, and now the old Toyota dealership I worked for uses them now, once I told them about it (they of course charged the same just made more profit). There prices are real cheap and the heads are very nice quality. My friend is having very good experiences with the one we put in his 3RZ-FE Tacoma. Check them out. (*Note: I don't work for CHI, but just trying to help out with a cheap set of quality heads, they're usually expensive!)
*EDIT: I looked it up real quick for ya (just below the italicized paragraph). Your profile says '92 3.0. They're 150 each. I can't remember what shipping is for these but I don't remember it being much at all. Here's the copy-paste information about the heads:
"This listing is for a rebuilt Ford Ranger 3.0 liter cast iron cylinder head. This head fits all Rangers up to and including 1999. For other models see our store listings. This head is completely rebuilt. It has been pressure checked for cracks and resurfaced. Guides and seats have been replaced as needed. New VITON valve stem seals have been installed. All heads are given a three angle valve job. All valves have been vacuum checked to ensure proper sealing."
Again, I don't work for them, nor do I get anything from them for the recommendations, its just real cheap and the heads are quality re-manufactured, you can't pass this place up if you need heads. I have personal experiences with them and that's why I can give such a high recommendation for them.