Antifreeze Leaks

I have had an ongoing problem with coolant leaks in my 98 Astro. I recently had the manifold gasket replaced (second time in 11 months), and the water pump replaced. The shop actually warrants their work for the life of the car, so if the same part fails, they will repalce it at no cost. However, I am not confident they have actually found the cause of the leaks - or at least the primary leak.

I still have leaks, but it is not usually visible. The only time I actually have seen it leaking is on two occasions. Both times antifreeze was running down the driveway. In other words an excessive leak. Both times the heater was running full blast and then was turned off. This once before and once after the water pump was replaced. I suspect the shop has not properly diagnosed the problem. It does not seem to be a continuous leak and I can not reproduce it at will. If it were always leaking, I would have to fill my coolant up every day.

I would like some advice as to how to diagnose this and get the real problem fixed.

What besides the water pump and manifold gasket have been checked?
I've had pressure leaks in the past that never left a drop of coolant on my driveway.

Not sure if your heater vaccum control valve is the same as my 95 Astro and 95 Safari, but that valve is notorious for leaking, especially when control is set about half heat. It is located on the pass. side, next to the AC compartment. You will see 2 heater hoses attached to it and a small vaccum line attached to the top of it... If they are the same. I know my year model is problematic. Hope this helps.
Look for water running out of it... with motor hot, you can even manually move the valve up and down (small linkage on bottom side of valve unit) and see if water streams down.