91 PA loose power with hot temperature

[HPman83]

Hi, I have a 1991 Park Avenue (156 000 Kms ~ 100K miles). When the temperature outside is hot (around 29-33°c) after around 1 hour drive on road, the engine loose power. At the begining the engine can't take over 3000 rpm and the problem becomes worse and at the end it can't take more than 1500rpm. It's just like if not enough gaz arrive in the engine.
When the motot is idle, it works fine.
I have changed the spark but the problem remains.
When I stop the car to cool down the motor, the engine works fine after.
The coolant level is OK.
And this problem never appear when the outside temperature is below 28°c.

Any ideas?

Thanks