The list is long but as already stated start with the simply 1st. fuel filter you could also have a clogged air filter runs fine till it gets warm then will need increasing air which she can't get. Don't forget the plugs
Just reminded me now Lynn. Some Pratt (or was it me been a prat 😂🤣) convinced me (a dealer) to put some of those plugs in with a fine centre diode where you don't have to alter the gap (forgot what they're called?) and my sporty conked out up on the North York Moors, near Whitby. Nearly getting stranded. Putting my old ones back in ( in my pack) got things moving again. Phew good job I'd kept them and was carry them at the time.
Could be the coils , I think they play up when they get hot , if it's running for first few miles and his fine , but then plays up when up to temperature, valve clearance up to spec ?