Ferrari may be grappling with a real reliability issue ahead of the 2019 season.
Last week, as he played down the Maranello team's superiority, boss Mattia Binotto said of the 2019 car: "I would like to see it more reliable overall.
"To win we will have to be the strongest and we are not yet."
Many observers thought Binotto's comments were a typical ploy to step away from the favourite role.
But former F1 driver Marc Surer said: "They had various problems in testing.
"It makes me wonder. If you only have one problem, you can solve it," he told Germany's Sport1. "But when they are different, that's difficult.
"I think it might have something to do with overheating. Ferrari has a very slim rear end, with few air intakes and a very small airbox."
However, Surer agrees that Ferrari has "a small advantage" over Mercedes in terms of pace.
"The advantage became much smaller in the second week," he said. "I'd say it was half a second in week one and maybe two tenths in week two, but they're ahead."
