When you're at highway speeds (or anything above 45 MPH), your fans are pretty much useless because you'll be pushing more air through the radiator at speed than the fans are capable of pushing/pulling through there, themselves. At that point, it comes down to the radiator, itself, as far as whether or not it's getting enough air through their, if there are any leaks, and if any of the cooling fins are blocked or heavily coated with rust or corrosion.
Do you still have the the plastic airdam strip underneath the radiator support? Without that, it won't direct enough air up through the radiator while you're at higher speeds to adequately cool things - the air just scoots right on past underneath the car. Just putting that thing on, alone, made a huge difference on both my '89 and my '84; before, they were able to cool adequately, but just barely so, and whenever it got a little warm from sitting around in traffic, it would take friggin' forever to cool off, but with that airdam in place, getting up above 45 MPH for about a minute or two would cool things off right away.
Also, maybe take the radiator upport mounts off and tilt the radiator over to take a peek between the radiator and the A/C condensor (if it's still there) and see if you have a bunch of debris built up in there. I've seen some ridiculous amounts of junk get caught between the two on several cars I've owned; one '94 Thunderbird I had had the whole front of the radiator completely covered with this crazy rug-like coating of HAIR that was blocking things terribly (made a MAJOR difference in cooling after I cleaned all that crap out). Either use a pressure washer, or a good ol' garden hose with a "fan" spray attachment, or maybe even carefully use a Shop Vac and get all the crud out of there ... bugs, leaves, fluffies, whatever.
Beyond that, if you peek inside the radiator and it looks awfully scaled-up, you might try giving it a good flushing, but if all of the above fails and your thermostat is good (you'll want a 195* t-stat for stock temp, or a 180* for a little extra cooling - I'd recommend a Stant Superstat), then you might just have to break down and buy a whole new radiator. I went through EVERYTHING on my '84 - fans, fan clutch, shroud, coolant flushes, you name it - and it kept overheating no matter what I did until I finally just replaced the radiator. Now, even with the crappy little 1-core or 2-core (whatever the OEM standard replacement one is), it stays nicely cool no matter how hot it gets outside ... although, because the radiator is a bit undersized for my application, it doesn't enjoy having the A/C running while I'm in stop-and-go traffic (but that's kind of an extreme situation considering it's over 110*F outside, lately).
