Sometimes the bot client can't handle more than 3-4 bots at once, and if it can't handle that many, it just usually crashes. Try using 2 bot clients to run 3 bots on each. That should do the job.
Pretty much what Haha said. That and make the java processes highest priority.
EDIT:
This will be the one time you see me recommending a premium bot since you're using PowerBot for mining. Sure they have great scripts there, but Kminer is the shit. I'd really recommend getting Kminer if you're going to be botting mining. Especially if you're doing all those accounts at once, you can make bank if you sell the accounts later on.
what do you mean by crash? your comp turn off? or just freeze

