is it a verticle core or cross flow? using a garden hose with the hoses off should tell you if its clogged if it doest back up with full volume from the hose, its probably fine. i had the exact same thing going on with my duster. one day i drove it for 3 hrs on the highway in the dead of summer and all of a sudden the temp started climbing at night while it was cooler. made no sense to me. that black funk sitting at the bottom of the block would start to loosen i guess with the higher flow of water at highway speed it would then restrict the flow in the rad enough to make the temp creep up yet would idle or low speed drive, with no issues. i did the panty hose trick on the upper hose for a while it sure did catch a ton of crap.
