When you replaced the plugs, were they worn? were the tips white? were the tips black? When the car refuses to go, does it smell hot? It seems to me, from your description, that the car is "bogging", which is usually too much fuel. Of course too little spark will result in too much fuel. However, if the car will sit and idle for forever, but then just won't go, the problem is usually low fuel pressure. You have enough fuel pressure to idle, but not to accelerate. This would be a fuel pump problem. I'm leaning towards a failing power stage, a near death fuel pump, or a clogged convertor...but answers to my queries may lead all of us towards the answer.

Chris