the huge swings from red line to 175 sounds like a stuck, or stopped up t-stat.

if it was me, i'd remove the t-stat completely, just to eliminate it as a possiblilty. you said you changed it, so its probably not a problem, but who knows, its a valve and its also real small, so if you had some kind of obstruction, like a big piece of gasket, it might be trapping it.

i'd also do a flush like this, in case there is some kind of debris, and to keep it out of the radiator.
remove t-stat and reinstall neck and hose.
remove radiator and turn upside down and flush. give anything in there a chance to fall out the top. reinstall.
remove upper hose from radiator.
stick water hose into radiator cap.
run the engine. its pulling fresh cold water through the radiator, running it through the engine, and dumping it out the top hose, onto the ground, not into the radiator. with no t-stat in, any debris should float out. temp will likely stay dead cold since your not recycling any of the water. this will also prove that your water pump is working. after a nice long flush where you've got nothing but clean water coming out, reinstall top hose and test drive it. leave t-stat out.

i doubt that the new fan is going to help. the fan's job mostly is to pull air across the radiator at slow speed and idle. at highway speed, its already getting all the air it needs.

your radiator could be stopped up. it sounds as if the heater core is providing all the cooling.

are you building up excessive pressure? a blown head gasket can put compression pressure into the cooling system and prevent water flow.

 

another thought.. if you or the previous owner live in a place with hard water and used that in the cooling system, the calcium will solidify out and stop up the radiator.

 

i do live with very hard water, so that was one of my first instinc, thats why i had the radiator flushed at a shop, they said that it was flowing fine. that is also why i have to flushed the entire system several times.

unfortunatley i do not have the space or tools to try your flushing technique, DHVAUGHN, i will have to wait til i get home.

i went ahead and installed a new fan and you were right, that wasnt it. not very happy about that. so hopefully i can limp home the same way i got here and flush the entire system. if that doesnt do it, well i guess i am in more trouble than i thought.

what are other signs of a blown headgasket. i havent noticed anything obvious. and if it wasnt for the cooling issue the truck is running like a top!

 

if the radiator has years of hard water in it, its likely that its heavily stopped up. it may be flowing somewhat, but the cooling capacity is likely shot. i'd replace it. the el-cheapo's are not very expensive. < $100
http://cgi.ebay.com/ebaymotors/DODGE...Q5fAccessories

 

Just tossing this out there:

I had a very similar issue years ago with a truck that had a swapped-in Chevy engine.

Long story short, after many water pumps/t-stats/and so forth... it was the fact that the lower hose no longer had a spring in it... so at 2000-2500 highway RPMs, the water pump was pulling enough to collapse the lower hose and restricting flow. I got a spring out of the lower hose on a junkyard truck, put it in, and all my issues went away. I still have a water pump on the shelf from that debacle, and its gotta be 10 years ago now.

 

I had that same issue with my 96 RAM. I bought three T-stats from A-zone and every one was bad. I put in three 180s and all were all over the map. Some were way up there and some were at 140. They were made in Mexico and were nothing but complete junk. When I took it back to them, the salesperson told me that the ones they sell are junk and lots of people find they don't work. I went with a better T-stat along with using Indian Head as the sealant for all mating surfaces. Problem solved. Part of the issue can be that water is passing around the T-stat if not thoroughly sealed at the housing and gasket junction. Water can blow around vs. going through. I believe there's also a certain way to align in it the housing for it to work properly as well.