Try to decrease "--scan-time" to 15 or something. If i understand it correctly the miner checks every --scan-time=seconds if a block was already submited, if not it continues to search this block, if it got submited it searches for the new block.
In other words: if your miner only checks every 30 seconds but someone found the block 5 seconds after you start mining for it, you would mine for 25 seconds on an old block.

I too see no reason to change the max amount of coins, especially after being listed on an exchange. Inflating a coin, without a realy good reason, after it's being traded would only angry investors as the per coin value would decrease as well as trust in this coin.

Someone was a bit faster in finding the block than you - bad luck. It happens from time to time.

From ~60 KH/s to 400+ KH/s in a few hours.
Someone must have started his botnet, now, that you've mentioned an exchange, lol.