I'm having nearly the same problem as the OP had:

 Tried to send  BTC to coinbase at the beginning of April from my old Bitcoin-qt wallet (0.8.6) but the transaction does not  even appear on the block chain. Interestingly, I was successful in sending coins from this wallet to the same address in March and February, although the wallet incorrectly says those transactions have zero confirmations.

I just wanted to check that the solution the OP was to try: install latest Bitcoin Core (now 0.14.1) and then run the "abandon transaction" command to get rid of the phantom transaction and restore the BTC that did not get sent - worked for him.

I was also considering trying the the -zapwallettxes startup option on my old wallet, but "abandon transaction" with a new wallet sounds much more appealing if it indeed works on transactions initiated before the wallet upgrade.

And just to be sure since I haven't upgraded in years - to upgrade to the new Bitcoin Core on my Windows 7 machine I just run the installer downloaded from bitcoin.org?

Thanks!