Chevy Beretta Coolant Leak

I have a 1996 Chevy beretta 3.1L v6. Recently it would almost overheat before cooling. Found the coolant was almost empty. Filled the coolant, it was gone in a week again. Noticed a mayo like buildup in oil filler cap and cover. Looks wet on both sides of the engine by the where the intake manifold meets the engine. I'm leaning towards a intake manifold gasket going bad but would like another opinion. The oil level does not seem to be rising nor is there any coolant in the oil pan. No puddles form under the car from a coolant leak. It has 118,000 and still runs great. I just replaced the thermostat while investigating the leak and it is not leaking from the thermostat housing.