B3000 heating & cooling system

I have a 1995 Mazda B3000 with 2001000 miles. The truck has been great over the years, One problem, I seem to have an internal coolant leak. Just to give you a somewhate quick synopsis, barely provides any heated air to the passenger compartment unless traveling at 60-70 mph, then it is barely warm & also the temp gage is on the cooler side. If stuck in traffic or a line, the temp gage will creep up to the "H" telling me it is overheating, If I rev the engine to a high idle it will go back down. Checked the oil, looks fine & has been fine over the course of this issue. I have bled the cooling system a number of times so I do not think this is the issue. In the last week or so & today, I stopped at a gas station to fill up, (just took a few minutes) when I started the truck it smoked like nobodies business, I pulled around back of the station, the exhaust feels extremely moist. No smell of antifreeze in the passenger compartment. I pulled all of the plugs to see if any of them would give me an indication of what cylinder has coolant leaking in, but no luck. Is it possible for just the intake manifold to have a leak & cause this problem with the oil looking fine? Oil being ok tells me the head gaskets are ok & a possible intake leak, please advise on what I should do before I get stranded please.