tjurgensen

I have a 1991 5.0 v8 that sat for 5 years before I got it. To begin with it had a slightly high idle (around 1100-1200rpm) and a noticeable surging. It would also overheat so I replaced the thermostat which was stuck. On the first drive after the replacement the temperature came up and dropped normally for a few miles then after awhile I could hear a very audible vacuum leak. The temperature had also come back up and didn't go down at all. After I parked the idle had gone up to between 1500 and 1600 rpm. There weren't any apparent coolant leaks that I could see. I don't have a clue as to what could be the cause or what i should do. Any explanations or advice would be greatly appreciated.

Thanks,
TJ