Why does transmission keep getting contaminated with water ?.

I will check the coolant reservoir, thank U. If so, do U recommend me replace the radiator ?.

Yes, and get all that coolant out of your transmission! It will destroy the clutches!! The rad. is the only place it can be coming from that i know of. Just drain and refill or have a shop do it. I do not recommend flushing the transmission at all!! With the lines off of the radiator, start the engine and let the fluid run into a pan while pouring fresh fluid back into the trans until fluid runs clean and clear of water. Keep a steady stream going back into the trans!! Like i stated, if not equipped, have this done.

I took Ur advice/replacing radiator. Do I install rad. first, and do I leave old filter and drain out fluid from tranny ? In other words, step 1, step 2 etc.

Do this before you hook the cooler lines back up after installing new rad, need some coolant in the system. Remember the fluid is going to start pouring out as soon as you start the engine!