Little Catfish Keeps Turning Over

My new corydorus hasbrosus (salt and pepper catfish) doesn't seem able to swim  properly.
It fights to stay in one place but when it moves it twists floats upside down for a few seconds before turning upright again.
I can't see any sign of injury or illness on it.
I have put him in a jug which is resting in the quarantine tank to acclimatise him.
What should I do next please

Thanks Sue.
I had it in a jug in the quarantine tank while I drip acclimatised.  It would sit at the bottom of the jug for a while then do the "whiffling" bit.

In the end I carefully tipped the jug into the QT and let it swim out but it disappeared completely.
Taken me ages to find it.
It was stuck upside down in the java moss on the cave. 
I had already tried the pea method while it was in the jug but it isn't feeding at all
There is no sign of illness that I can see.
This is one of two I bought recently.
My other two corys are missing in the big tank too but it is possible they are hiding.

I'll just have to keep watching it

I'm sorry to hear that. Since it was new, it was most likely a bit iffy when you got it, not caused by anything in your tank.

Very sorry to read of this, Sanjo.

Thank you all. I have buried him in the garden, he was so pretty too and never seemed as if there was anything wrong with him.

All I have to do now  is find the other two!