John Deere Stx 38 deck keeps falling off what can you do?

My JOHN DEERe STX38 black deck would fall off every time I hit a dirt mound or unseen tree root. The two steel bars would jump over the washers above the top rear of the deck and the deck would fall to the ground. The last time it did it, it broke a one month old $40.00 belt. I fixed the problem by removing the two bolts (one on the left side, one on the right side) and replacing the two washers with larger 2" washers. I found some 1/4" x 2" washers at Ace Hardware and drilled the 1/4" holes out a little larger to fit the bolts. I actually used two of the new washers together on each bolt since two of the new ones were the same thickness as one of the old ones. The deck no longer falls off. Total cost, $1.20 at Ace Hardware.