2002 Land Rover Discovery 2 Antifreeze leaks

My 2002 Land Rover Discovery has been having coolant leak problems since the summer. During the summer it was leaking on to the engine. I took it to the mechanic and he replaced a head gasket and it was running fine for a while. Now there is a steady antifreeze leak from under the car. You actually have to drive the car for a while at a certain speed in order to create the leak and the superheated steam. Its dripping on to the exhaust system and also seems to dripping from the transmission. I can't really get up under the car properly to see if its a hose. Took it to the mechanic the other day. He let the car run, but he didn't see a leak. 30 min after we drove it you start to see the smoke. I drove it today just see if I could get under the car and see where the leak was originating from. Can't really tell. There's one spot where there's a steady stream. Then there's another spot where there's a slow drip. The coolant must be spraying from something. Got any thoughts before I take it back to the mechanic?