Overheating. Help its driving me crazy

I have a 1996 Buick Century with te 3.1 6 cyl engine. I was overheating I changed the radiator that was bad thermostat and the water pump that was weaping , the radiator cap, temp sending unit, and I checked the the fan and it turns on fine.. and it continues to over heat when 70 mph or on long drives over 30 minutes... It also it sends water back to the overflow bottle as soon as I turn on the engine. I saw today a little oil on radiator cap and radiator neck. Could it be a HG problem...? But there is no water in the oil. Please help me anything helps.

Thanks. Mike

After closer inspection I see that there is seeping in the intake manifold gasket behind the power steering pump. It could be that. I'll let you know the result after I replace it.