Water damage, but now temperature warning - what next?

My Son had a tumble while carrying a backpack containing of all things 1 litre of juice, and his iPhone 4 A1332 model.

Due to the backpack being waterproof, and the bottle of juice getting smashed the iPhone was left submerged for a few minutes while we sorted him out.

I stripped the back from the phone, disconnected the battery and left it to dry. I got a thirsty bag on order and never touched the phone for 2 weeks.

After re-assembling the phone, everything seems to run fine for about 5-10 minutes then it warns of temperature and the phone needs to cool down. If you don't turn it off it runs slower and slower until it hangs.

I've disconnected the battery again and put it back in the thirsty bag.

So - am I right in thinking due to the sugar in the juice, even though the phone has probably dried, the sugar remains on some surfaces, causing over-heating and/or sensor errors?

What would the next step be? I've seen mention of ISO Alcohol (95%+) - however do I submerge the phone in this, or strip it right down and clean each part individually?

It's become a bit of a project now. I've bought the iFixit tools and repair book, and having worked on PCs years ago already have the ESD mat and wrist strap.

Any advice and pointers really appreciated.

Thanks.