Adrian Sutil says Sauber's Spanish Grand Prix upgrades 'didn't work' and the Formula 1 car remains extremely difficult to drive.
Sauber has endured a tough start to the 2014 F1 season, and is - along with Marussia and Caterham - one of only three teams still to score a point.
"The update in Spain didn't really work. The only thing that really worked was the weight reduction," said Sutil in Monaco on Wednesday.
"The biggest problem, and why we are not performing well, is the drivability of the car and the strange driving you experience with it.
"You can't control it in certain races. In Barcelona, you went from tricky-to-drive to undriveable and tyres looking up, front, rear, oversteer, understeer, you have everything.
"And this is the worst thing you can have, you don't know how to solve a problem, you just change it from one end to another. Then it's just multiplying all the problems."
During winter testing, Sutil found the Sauber's braking system difficult to get to grips with, but he now suspects that more fundamental problems with the car might have caused this.
"It's still a bit strange," he said of the C33's braking behaviour, "but I also think it comes from the loss of grip when we enter a corner.
"It might be somewhere else where we have to look first. Maybe the brake system is doing the right thing but we just haven't got enough grip."
The German admitted that he had been increasingly pessimistic as pre-season testing went on, and felt that mood had proved justified.
"To be honest after the first few tests I thought, 'that's going to be a hard start.' But you always think in a positive way," said Sutil.
"But I had a few worries before the first race, to be honest. We were not ready and the car, from a drivability [point of view], it was not where we should have been.
"Still that's where we are at the moment and we have to improve, we have to try to make it better. But it's one of the most difficult starts.
"It's very, very hard to get out of it at the moment."