70 has a problem with the temp creeping up

Running a 2 core 340 rad in my 70, 180 stat, 18" viscous fan, new water pump. Iron headed 360, mild cam (.480ish on both sides, can't remember exactly). Ever since I got it together it has had a temp creep problem. Running down the road at 55 mph (about 2600-3000 rpm, no tach yet), it runs cool, but when I slow it down to about 45-50 mph it starts to heat up, slowly, but consistently. It also does it at idle, but only after I've been moving. If I hammer on it, it will cool down also. At first I thought it was the summer time when I finally got it together (ambi temp was 95-100 deg), but now we are having good temps (70-80 deg) and it still has the problem. It was worse until I advanced the idle timing to 16 degrees (IIRC), but it still is there. I was thinking about buying one of those aluminum radiators. Any ideas?