Granted that it was on a V6, but have heard a rumor of someone who inadvertently switched the lines going to the expansion tank when he first got his red Trooper, but before he had put on the ARB bullbar, etc. Since that unnamed person had just purchased the vehicle used from a dealership, he took it back to the dealer. The dealer had the vehicle for a full day each of two times and was unable to determine the cause of the problems that sound identical to your symptoms.

The dealer never found the cause of the problem but the owner eventually did. That discovery has led to steadily downward spiral in the owners character, epitomized by his current role as a moderator, but he shall remain unnamed.