This one turned out to be a fault on the heater !!! there is a tee off on the vac control pipe to work heater ( recirc ) flap.
In one position it was leaking and lost vacuum causing the problem, In the other position it was OK, typically in that position when came in so Vac tested OK, but eventually twigged what was happening. Just plugged the pipe
you need to be 100 % sure about leaks, rember a smoke machine does not work at the same pressure as a turbo
The vac regulator valve (common and difficult to prove) and wiring to it (under rad ) need to be checked /eliminated also