stalling problem - 75

Hi, can anyone help with diagnosing this problem on my AR75? Its been
to a mechanic twice, but no luck in fixing it - in fact I suspect the
guy thinks I'm making it up! Here's the symptoms:

(1) Intermitent
(2) Idles without any problem
(3) But when I try to move off (release clutch, depress
accelorator), the car will strain and then begin to stall. (It has the
same feeling you'd get when trying to start in 5th gear.) If I then
depress the clutch it will happily idle again, but if I keep trying to
move it'll stall.
(4) There's just enough power to start the car rolling if I'm on the
flat, but if the engine actually needs to do some work (ie climb a mild
slope), then it hasn't got the power and will stall.

The mechanic said that it sounds like fuel starvation, but he's traced
through the fuel system and hasn't found anything, and by the time its
been towed to his shop its fine again!!

Any clues?