The fact that the Grinder allows you to dive into its Jython script holds a lot of power to customise the manner in which you'd like to control an "agent's" behaviour.
For example - if you'd like to simulate dropped network connections, say your agent has 100 runs. you can generate an array of random numbers [between 0-99] and for the runs matching any of the random numbers, you can instruct the thread to abort.
However, having said that, the conditions you wish to simulate are more like tweaking the behaviour of the server you're testing. What i provided you is a work-around which may or may not fulfill your expectations. The Grinder as a tool doesn't really offer much to influence the behaviour of the server [to be tested]. it's more like a tool to control the user load and behaviour.
Hope this helps.