Driveability Issue
 

I have a 1990 Turbo A/T, at 60kph at 1600-1700rpm with a steady throttle the rpms will drop down to about 1000 and then will pick back up. The car will not stall but will do this untill I speeed up or down, also will only happen when up to temp. I have checked and replaced vacume lines on it. Any suggestions?