Is the Testnet stable? I have been recently playing with the testnet (using bitcoin-qt -testnet). I have been able to mine a few coins and do some test transfers, but I have found some problems:


The coins appear sent, but a second, receiving client never saw them.
When launching the client some days later the mined coins (and all the blockchain data) have vanished.


I have read we are now in the third testnet. I have checked my .bitcoin directory and there are two tesnet directories, one named testnet and a second called testnet3. 

Am I connecting to two different networks?

I thought there was just a testnet, but now I have my doubts. If there are many testnets or if the testnet can change, how can I make sure I can still check my old transactions?
Posting my comment as an answer

Make sure your client is up to date... it's possible your wallet in the old testnet got coins that is separate than the more modern testnet3.

Each version of bitcoind has the possibility of being associated with a different test network.  It would be interesting to see if there was a matrix listing releases against test networks (and why the new network was created).

I assume that it's possible to launch older versions of bitcoind and see each testnet independently of each other, but that independence means that there is no way to transfer coins from one network to the other.  (It is test after all, and hopefully one didn't pay for those coins)