not sure if you are mining this coin, the blockchain im on currently has 430 blocks and is mining ok (found 2 blocks in last 5 minutes)

Sorry, I forgot about the timeout. The command you want is "walletpassphrase 600". Bitcoin-qt should show a lock icon in the lower right corner of the main window, it will show as an open lock once when wallet is successfully locked. It will automatically lock again after 10 minutes or when you use the "walletlock" command, whichever is first.

Is your wallet encrypted? If so, try this:
Typing "walletpassphrase 600" doesn't work if your passphrase isn't "600". Try the above, but replace the red text with your actual passphrase.

If your wallet's not encrypted, you can skip the "walletpassphrase" and "walletlock" steps.

If that still doesn't work, I'd suggest a rescan of the blockchain (close bitcoin-qt and run "bitcoin-qt --rescan" on the command line, it'll take a while to start). Try rebooting your computer too, that can solve a lot of problems.

This is one interesting case. An exported private key that cannot be imported and a wallet that doesn't sign, that does sound like a corrupted wallet. Going to follow this.

Then I think there was some error while exporting that private key.

You should double check steps described by: Kouye
https://bitcointalk.org/index.php?topic=354762.msg3810656#msg3810656

Or try running this command before those described by him:
walletpassphrase "your walletpassphrase here" 600

How does that private key look like?

Does it begin with 5 and have ~51 characters?

I recommend you to go to https://www.bitaddress.org/ choose tab "Wallet Details" and enter the private key you have.

Check if you can decode it there. If you can, try importing to a blockchain.info wallet what you get under "Private Key WIF".

If it still fails, let us know in what step and with what output.

Electrum might not be the best choice, seedless wallets did not allow importing, last time I checked.
Do you get the same error on a blockchain.info wallet?

Go to help=>Debuging Window=>Console and type
Find one with an "amout" value big enough for your needs, copy the corresponding txid.
Go to blockchain.info, and paste the "txid"
Check the public address of yours that received the funds, copy it.
Back in the console of QT (you may need to use walletpassphrase before if you encrypted the wallet):
Copy the result (do not share this with anyone, don't post it here, for example)
Back to blockchain.info
If you already have a wallet there, just import the private key using the import feature (and pasting the result of the dumpprivkey command). If not, create a wallet before importing.
From the blockchain wallet, send the coins to wherever you want.
Once the unspent output is clear, you can remove the private key from blockchain wallet (you need to archive it first, I think).

If that fails, please specify at which step it did, please.

You could try a -rescan, though I doubt that will do anything.
You could try Armory, which will give you more details.
You could try extracting the keys and importing them in another wallet.
You could try importing the wallet.dat in blockchain.info (https://blockchain.info/wallet/import-wallet)

Well depending upon "how small" it may be possible that you don't have enough to pay a tx fee (although 0.08 BTC would seem quite a lot even if you have a *lot* of small amounts).

Perhaps try a smaller amount (say 1.0 BTC) first to make sure that this isn't the case.

That is unusual then - if you don't mind me asking were the BTC you received in just a few txs or were there many small ones (this could affect things although hard to believe by that much)?