Heikki Kovalainen has described the Caterham CT03 as being "on a knife edge" after his first experience of the machine in free practice for the Bahrain Grand Prix.
The 31-year-old, a veteran of 109 grand prix starts, has been drafted back in by Caterham as a reserve driver to evaluate the car and planned upgrades.
But even though the car is similar to the machine he raced last year and is built around the same monocoque, Kovalainen found it much harder to drive.
"Straight away, the feeling with the car was that there isn't any margin for error through a corner when you lean on it - it's on a knife edge," said Kovalainen.
"It's more unbalanced than I experienced last year and it's more difficult to drive, more inconsistent."
"To a certain extent, it is not a surprise because of the way that the car has been modified in terms of ride height and in terms of some stiffness."
Kovalainen drove Giedo van der Garde's Chinese Grand Prix-specification machine in Bahrain.
The team tried some updates on the sister machine of Charles Pic and the Finn is scheduled to drive the Caterham when the rest of its major upgrade is introduced in Spain next month.
He believes that set-up changes could ameliorate the handling problems as although he suspects the Pirelli 2013 rubber is a contributing factor, other teams have achieved a far better balance.
"Part of it is solvable with set-up but when we improve the car and put upgrades on, the one area that we need to improve is the back end," said Kovalainen when asked about whether progress can be made with the setup.
"It needs to be stronger. We are not the only ones and the tyres are part of that but it could be improved.
"It would be interesting to understand why some cars have better balance than other cars.
"It's not just the downforce, it could be on the mechanical side as well."