Intermittent hard to find fault

[Glyn.Stanworth@epsedin.co.uk (Glyn Stanworth)]

Hello mailing list,
Is anyone familiar with the problem I'm currently experiencing with my motor, or have
any theories as to possible causes. I suspect that the problem is a freak which is
not specific to the car type but I don't really know.
I own a 1987 VW SciroccoScala which has developed an intermittent engine performance
problem which is becoming more persistent irritating and fuel expensive, I've 
visited a number of mechanics and picked the brains of numerous friends to
no avail.
When travelling on the motorway for a length of time (20mins - 1 hr) at fairly constant
speed (approx 80mph) the engine can begin to lose power. The severity of the problem
is variable sometimes I just lose the ability to accelerate, sometimes the car just about
grinds to a halt forcing me onto the hard shoulder. Whenever the problem arises
engine speed drops suddenly and sharply if I slightly lift the gas pedal, pressing the gas
pedal back down then has little effect 
In the less severe cases the problem sometimes goes away for a time and then reappears 
later, more often I end up stopped. In this case the idle speed is very low 
and the problem can be partially fixed by running the engine at high revs in neutral.
It can take a while to get the revs high enough 6 -7000, the engine will then be running 
irregularly seeming to splutter occasionally - eventually the problem disappears suddenly
almost as if a blockage is being cleared. 
The engine has been recently tuned, plugs are fine, new fuel filter, I've cleaned up
the air filter area, and tried running the car without an air filter but still the problem
persists. I've had people look at the carburettor but they find nothing wrong.
Diagnosis is made difficult because the car runs very well around town with good
fuel economy, but badly after a while on the motorway with very poor fuel economy.

If you've got any experience of this problem or any idea of where I might look for
a solution I'd be very grateful to hear from you.

Cheers

     Glyn Stanworth

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