Power drop in 1989 928 S4

My Porche has had a power drop, often when rpm's drop suddenly as in coming down a hill.  The idle gets rough and smooth acceleration is lost

The only cure is to pull over, restart engine and the car resumes full power

We have checked everything we can think of and fixed it.. ie rotor, wires, fuel pump, gas tank cleaned and filter cleaned etc

We did a smoke test and found numerous vacume hoses were leaking under the intake manifold.. could this be the cause?

I have maintained the car very well, all systems are great except this issue, and after I fix this, I plan to sell it.  The car is in very good condition. I have no idea how to properly price

it ... can you assist by giving a range, for my zip code, 07456> ?  Many thanks John Crotty