87' sami   new rad  still heating up on the freeway   thinking it could be water pump?   got my 6.5 to 1 this week hoping that will take the load off trying to keep up on the freeway?

Block passages clogged up maybe?  If anyone has ever ran any block/radiator seal in it, it very well could of clogged up the block.
The prop fins would have to be almost completely gone for the water pump to stop circulating coolant....I've never seen any in that bad of shape, but it could happen, I guess.  Might not hurt to change it out anyway, if you haven't ever changed it......pumps are cheap, just a PITA to change out.
Brand new from the store radiator, or just a different "new" radiator?
Proper coolant mixture? (likely not an issue, even if it is incorrect, but just trying to think of possible causes).
Does it cool fine at low speeds, but just not at highway speeds?

For how cheap they are, can't hurt to change the thermostat again, too.  Or just take it out for a couple days even.

Fan clutch?  Rad cap?

Had the same problem with my Sidekick and it turned out to be the new generic temp sensor I installed from a parts store. Got a factory sensor and all was well ....Good luck

If it gets hot only while at cruiseing speed you should check your lower rad. hose. Sometimes it will collapse and not circulate enough coolant. You can check it by letting the motor heat up and hold the rpm at 2500 and look at the hose to see if it is collapseing.

   Kevin

rad and cap brand new     temp good at 55  over that starts  to heat up       getting new term in the morning  i got a new job at an auto parts store cost +10% on every thing          had block flushed all looked good?   maybe just pushing to hard with 33s   t-case gears go in next week maybe that will help

With cooling systems, it could be....
* T-stat (check this in boiling water with a thermometer to verify proper operation)
* Hoses (make sure they aren't collapsing)
* Radiator (make sure it flows freely)
* Have you felt hoses to make sure they're a little firm, and warm on top, cooler on the bottom?
* Ignition timing (I've seen this make headers RED hot if timing's off bad enough)
* Blocked passages (don't know how to check this)
* Pump (usually there's a lot of seepage out of this little bypass hole when it's bad. Also check for any 'free play' or wriggle room in the shaft)

All fixed  had bad therostat  put in electric fan just 4 fun     Now run so cool havent even had to turn the fan on even i traffic i like that   now its time to go to the beach