Chev Monte Carlo Z34 1997 40,000 miles 3.4L

A leaky water pump was replaced a couple of days ago, and I noticed the temperature was erratic. It read 260 degrees and I stopped to check and the car just didnt seem to be running hot. Yesterday I went to take the car back to have it corrected, and when I got into the car I noticed the temperature gauge was sitting at 260 degrees, even though the car had been parked in the garage all night. The dealership wasted two hours of my time and gave me a song and dance about needing more time to diagnose the problem and letting them have the car for a day. This doesnt seem like rocket science and I refuse to believe it takes two hours, not to mention a whole day to diagnose what should be a simple problem for them. Can you give me your idea of what the problem could be so I can help out these overpaid mechanics? This cost me 388 dollars.