It's possible that you haven't taken the card board out from in front of the Radiator. I know a member here who puts a piece of cardboard in front of his radiator during the winter to keep the engine warm. (one to many fan mods) He failed to take it out one sunny afternoon while we were cruising and got stranded

Seriously though, possibly a pump going bad? Others with more knowledge will probably add in their $0.02 I hope you figure it out soon though as it looks like it may be a warm one this summer.