I've seen this too. This is the orange colored "extended life" coolant that's supposed to go 100,000 miles without changing. From what I've seen, the thing to do is drain that stuff out, flush like crazy, and replace it with the regular green colored coolant. If you're already experiencing problems, you may want to do several flushes until the drained coolant looks clean. (As in, flush, replace, drive for a while, then drain again and check if it looks clean or rusty.) I have a 96 Blazer that hasn't had any problems yet, but I'll be flushing this month.