Condor Pista chainset (?) issues

I'm trying to prepare my fixie for the winter. I've had it for 4 years and bought it second hand (from first owner, not used much). It's 6 y.o. altogether. I have previously replaced and/or fixed all sorts of small bits that have worn off, but this is the first time I have an issue with the chainset etc., so I thought asking a crowd of specialists is a good idea. The issue is as follows:

• when I crank hard, I get heavy cracking sounds from the chainset - it seems to always be the same spot, but every OTHER revolution of the pedals
  
• when I tension the chain properly, I get PLENTY of resistance in the system - so I can still go places, but it's noticeably harder
  
  

Yesterday I spent most day cleaning the chain, the chainring and the sprocket. Now all shiny and lubed up, the sounds are still there, only a little clearer. So I wonder whether I should:

1. Replace the chain - this is clearly overdue and I can SEE it doesn't fit the chainring as well as it should - although it NEVER slips or anything like that
   
2. Replace the chainring/chainset
   
3. Replace the sprocket
   
4. Learn how to measure the chainline and make sure this is aligned (despite that this is still the original Condor setup and should therefore be OK?)
   
5. Replace the bottom bracket
   
   

Any ideas on what combination of the above I should start with? If any photos would be helpful, pls let me know.

Thanks in advance for all shared wisdom!

Al

If it's every 2 revolutions of the crankset, then I'd personally double check that it isn't something on the wheel that's making the creaking. Slightly out of true wheel rubbing the brake block in the same place each rotation, or a knackered bearing in the wheel that makes the noise in the same place each time. Try spinning each wheel with the bike off the ground to see if the noise is the same.

If not, try taking the chain off the crank and then listening for the noise spinning the crank. If you hear a noise I'd suggest BB, if you don't, my first port of call would be changing the chain - very cheap and if it's very old or very worn there might be a stuck link that's causing the noise.

Also worth saying, check that your chainring bolts (the bits that attach the chainring to the crank) are tight enough.

Hopefully one of the above should sort it