Former F1 champion Nico Rosberg has been left astonished by Mercedes' struggles at the Emilia Romagna Grand Prix.
Lewis Hamilton and George Russell were unable to get within five seconds of Ferrari duo Charles Leclerc and Carlos Sainz in the sole practice session ahead of Friday's qualifying at Imola.
Seven-time champion Hamilton scraped through Q1 by only four-thousandths of a second but with rain arriving in Q2 and with both Mercedes drivers outside the top 10, Sainz crashed at the second Rivazza to trigger a stoppage.
Rain fell during the red-flag period to render the end of the session meaningless, meaning both Silver Arrows missed out on the top 10 shoot-out for the first time in a decade.
Speaking to Sky Sports F1, Rosberg said: "It is unbelievable to see how Mercedes is struggling.
"It just doesn't seem to work at all, that car. You see the jumping with George Russell, that seriously gives you back and headaches just watching that. It is unbelievable.
"You get to the end of the straight and you need to slam the brakes in that condition. You have no confidence at all to attack the car when it is bouncing like that. Really fighting with that incredibly.
"They can't get to seem to understand what to do to sort it out aerodynamically. Incredible to see them back there."
Mercedes have "big correlation issue"
Both Russell and Hamilton have continually experienced porpoising throughout the season so far, with no fix seemingly in sight.
On how a solution could be generated, Rosberg added: "The drivers are not going to be too involved in this issue, it is going to be the aerodynamicists.
"It is about data gathering on track, but it is always very difficult to gather accurate data, especially when you have such a fast bouncing movement of the car because the air is going to be all over the place under the car.
"The problem is they don't see that problem in the windtunnel because the conditions are stable so it is a different environment.
"There is a big correlation issue and that also makes it difficult to understand the problem and find solutions."