Might not be fuel. Try changing the rotor arm and distributor cap. Only about a tenner for new ones and an easy job to do. I had an old Toyota years ago and that started spluttering, my brother was convinced it was fuel. I changed rotor arm etc and it ran fine after that.
how old are the spark plugs and HT leads
start with new leads and dizzy cap and rotor arm if it has them
it may have 4 separate coils though and they are prone to failure
run the engine at night with it cold and the bonnet up. look for sparks that would indicate ignition failure
do you run the fuel tank near empty all the time ??
could be full of rust and need a clean out and a new fuel filter
if you are doing short journeys, blank off the bottom of the radiator to allow it to warm up quicker
also give it a good long run occasionally
changed the air filter recently ?
if you do short journeys it may be blocked and full of sludge from the crankcase breather
Could be low fuel pressure (shitted up old fuel filter) dodgy old petrol, contaminated with diesel, paraffin, etc.
Weak fuel pump. The old mechanical ones would wear so much they were incapable of priming themselves if the vehicle was run dry of petrol and you'd have to pull the feed pipe, then pour petrol in from above to get it going again. A vehicle with a pump in this state which has never run out of petrol will still start and drive but may not have adequate fuel delivery. Electric fuel pumps are usually relay controlled. Some relays fail intermittently and car might refuse to start, but if you're used to the car, you might hear or not hear the leccy fuel pump starting up when you first turn the key, as a clue to whether it's working or not. When they fail whilst car running, they tend to just cut out.
Earlier models, ie part way through production run for Fiesta Mk2, they changed from points to electronic ign. If it's old enough and has points, it can be duff condenser, or knackered points. They gradually close up with prolonged use, and it causes a poor spark and puts the dwell and timing out.
Later cars, dirty or worn injectors (chuck injector cleaner in tank, run engine whilst lightly tapping each injector with toffee hammer or blunt end of a drill bit to loosen the dirt that builds up) loose cam sensor, crank sensor, (if they're able to move at all, they can cause uneven running, flat spots and or misfire) dirt in throttle body (upsetting throttle position sensor) which puts some ECUs into limp home mode so they ignore signals from lanbda sensor, run rich and sluggish, blocked cat, water in fuel, although this will usually make the car undriveable unless it's only a tiny amount (can get in through rusty filler neck where these are made of steel).
Insulation breaking down on HT leads, wrong ignition timing, wrong valve timing, ECU mapping gone stupid, bits falling off, fail the MOT, scrap it etc.
I had a Mk1 Fiesta many years ago and when I took it in for a servicethe mechanic identified a problem with the vacuum advance - he mentioned the car being sluggish when he tried to accelerate. He reckoned I would need a new distributor and other gubbins. I had a word with a colleague who worked in MTSS - he had a look and re-adjusted the handbrake. Problem solved.
The garage next door had a customer with a mk1 1100. He brought it in for an oil leak to be diagnosed and fixed. The cause was a hole in the aluminium timing chain cover, due to a worn timing chain rubbing against the inside of it for years. He'd ignored the noise until it started losing oil. We gave him a cover off a scrap engine. A new chain was about a tenner at the time, and an hour's labour to fit.
I always pay a motor engineer to maintain my cars
works out cheaper in the long run
regular servicing and used for long runs, local stuff its a bike or shanks
only breakdown so far a broken clutch pedal,and that was easily fixed
200K on the clock 52mpg and burns neglible oil so far