Hi
hopeing ther emay be a few ideas out there as to what I should look at first
went for a run out, ~90miles, no problems.
came home and sat at a steady speed for some time (unexiting dual carraigeway)
foot down to overtake, and rather than accelerating, lose power!
back off, shift down and normal service is resumed
all was fine, and seemed normal for another 20 or so miles and the problem repeated

crap in the fuel line blocking pump/carbs?
or something more serious?

Possibly carb (or intake) icing ??? Are you running carbs? Sometimes a give away is a very sooty exhaust and if your car's not too noisy a whistling sound from the intake. People never seem to consider this in summer! Any temp between about 6 - 25C with high humidity can give this...
Other ideas maybe fuel tank not venting properly.

sounds similar to my problem...

please tell what setup you are running...

fiat 1.6l twin cam engine (muira?)
with twin webbers

also had the same experience but im running injection and i have been trying to solve the problem. Can be a bit embarassing when you try to get away quick from the lights!

sounds like some kind of fuel pressure/starvation issue.....

is the fuel tank vented?
how is fuel supplied to webbers?
does this seem to happen after you have been driving for a similar amount of time?
is it possibly related to the level of fuel left in the tank?
how is fuel drawn from the tank? dip tube from the top or tube fitted at the bottom?

Exactly the symptoms I had on a company Astra some years ago. After much diagnosis and the replacement of the EGR valve, they decided to replace the complete engine bay wiring harness - luckily just before doing that, a good old-fashioned mechanic had a hunch, changed the ignition leads and the problem was cured.