The 2nd part of your post regarding the A/C being on and the fans coming on with it, makes me think it is an otterstat problem.
Anytime the A/C compressor is engaged (compressor clutch snaps in), the cooling fans upfront should turn on. Then, when the air conditioning compressor cycles off, the cooling fans shut-off. This is normal and in cooler weather where there isn't much air cooling to do, the A/C compressor will not stay on for very long.
The other way your cooling fans are meant to come on is via the otterstat. Sounds like that portion isn't working as intended.
Tell me, what were your cooling system problems that led you to replace the radiator? Not assuming this wasn't a good or necessary thing to do, but perhaps it wasn't the thing you needed to do to get it resolved?
You can test the otterstat functionality pretty easily by jumpering the wiring harness side of the two leads coming to the otterstat together and seeing if the cooling fans turn themselves on. Engine doesn't even need to be running to test it this way, just have the key in the ignition and turned to accessories.
Picture of the otterstat and where to jumper it is here:
http://dmctalk.org/showthread.php?10...l=1#post166051