I've not heard of a clogged radiator before on a GL1500, and certainly not using the Honda premix. Your radiator should be fine. The first thing I'd look at is the thermostat.
I had the same problem on my '99. The temp seemed to run a little hotter than before and the fans kicked in more often but didn't run for very long. I wasn't losing coolant but I could smell it as though it had boiled over onto the engine. I determined that the thermostat had failed and stuck shut since the engine warmed up quickly but after a ride the temp in the overflow tank was cold. Oh, and good luck finding it! It took me three days to change mine. Honda burried it under a lot of motorcycle. It is on top of the engine in the front. I removed both radiators and the right side cooling fan to get to it. Honda probably used a 195 degree thermostat and although it will look a little different, the thermostat for the '98 Honda Prelude works just fine, is cheaper, and available at any auto parts store. Be sure to pick up a new gasket also. The type that fits around the edge of the thermostat. Here in SoCal the summer sometimes can get a little hot and I put in a 180 unit. It does run cooler and I noticed the temp needle reporting this. I also ran plain water for a few days to make sure I hadn't cracked the thermostat housing when I tightened the bolts. The housing seems to be made of resin. After I was sure there were no leaks I replaced the water with a 50/50 premix.
Ed
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