Lewis Hamilton called the return of porpoising a "concern" after a tricky Belgian Grand Prix.
The seven-time world champion finished fourth at Spa-Francorchamps, though did pick up the bonus point for the fastest lap after pitting for new tyres in the closing laps.
However, Hamilton's race was limited by an old enemy that Mercedes had had under control until last weekend in the Ardennes.
"We had big bouncing this weekend, so we are back to the bouncing like we had last year," Lewis claimed post-race.
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"So at the end, I was keeping the Ferrari within a few seconds but I couldn’t get any closer to that battle.
"Then to get the fastest lap at the end, it was nice to have the gap to the car behind, it was decent. But it was gusty today and very difficult out there to keep it on track."
More worryingly, Hamilton's porpoising problems came despite running a lower downforce setup compared to teammate George Russell, who finished down in sixth.
With the Briton conceding Mercedes needs to find answers before the next race in the Netherlands.
"They don’t know and to me, it is a concern, but we will work through the data this week and try to figure out what we do before the next race," he said.
"But I don’t really have much more answers for you. I know what I want and I am praying for it, I am just waiting for the day that we get it."
Mercedes did arrive at Spa with a significant upgrade package, including a new floor and sidepods.
And team boss Toto Wolff thinks it is plausible the added downforce could have triggered the new porpoising problem.
"The main limiting factor was the bouncing - the car was just bouncing literally on every straight," commented the Austrian.
"Even Blanchimont was a corner where Lewis had to lift, and it is an easy flat normally. So, you’re bouncing on the straight, you overheat the tyres under braking, and that is a vicious circle.
"We’ve got to analyse it," Wolff added. "There is so much hard work that’s been going on for the upgrade.
"The aero guys, and the operations to deliver that upgrade, you’ve got to take your hat off to them.
"I think the floor could have been the reason why we’ve been bouncing, but we’re going to see it in the data.
"And maybe there’s something that we need to find because I still believe the direction that we’re going now is the right one."
