Left on a 200 mile trip in morning at 75 degrees ambient. After a while, turned on AC. No trouble on trip down. On trip back, at appx 85 degrees at 75 mph on hwy, car felt like it was pulsating, and losing power, eventually, would only run 40 mph. Temp gauge was a little above middle. Stopped, raised hood, looked for anything unusual, found nothing. After 5 or 10 minutes, started car and it drove fine again at 70 mph. After about 50 miles, same thing, pulsating, and car would only go 40 mph. Stopped, looked under hood again, checked water level. Returned to highway, turned AC off, heat on full, with fan on full, and all windows down. drove another 100 miles at 70mph with no issues at all. I think I need a new radiator, but am not aware that performance is affected at slightly above middle level of temp gauge. No check engine light. Any ideas?