1989 Chrysler New Yorker overheating

I am a mechanic in England and have been for 15 years but this has me puzzled. My 1989 3.0l new Yorker can sit in the driveway running for over an hour ,the temp gauge goes up ;the fan kicks in ;the temp drops all working properly, however when the car drives it can only drive for about 20 minutes before the gauge goes up (gradually) and the coolant boils over. There are no bubbles coming up through the radiator and the hoses are not overly pressurized.When driving the car it does feel like there is something holding it back on acceleration but all the brakes are free and working correctly.The A/C blows hot only(because the vacuum operated valve has been removed due to leakage).can you please help me because I have to go back to England soon and I need to leave my Girlfriend with a working car. I would greatly appreciate any advice you could offer me or information about a similar problem you may have already come across.Thank you in advance Paul