1996 Toyota Camry occasional white smoke noted from exhaust

I am so frustrated and worried at the same time. I have a 1996 Camry which I bought in Feb 08. It had miles on it. After 2950 miles, the check engine light came on and I took it back to the dealer where they looked at it and said it was the spark plugs and gladly replaced. We brought the car home. It was driven about another 100 miles or so and then made a HORRIBLE grinding garbage disposal sound. It was towed back to the dealer/mechanic where they proceeded to tell me and show me that the engine "blew a rod" . They were very helpful and at NO CHARGE replaced the engine with a used one. We brought it home drove it for a couple of weeks and noticed that it was leaking oil all over the driveway. Spots here and there, no real pattern. Back to the shop. They replaced the oil pump, the gasket and still leaking. Ahhhh another used engine was put in and no leaks were found. We have had the car home now for about 1 month and it drives well. It went for its emissions testing and passed with flying colors. it doesn't rattle, tick, ping or click, but I have noticed on rare occasion that white smoke puffs out from the exhaust. There isn't any trouble with the way the car drives. The check engine light doesn't come on. I think I will just die though if I have to take this car back again. Any ideas? Please?
Thanks
Pulling My Hair Out!