Fiat Uno. Intermitant start-stop

Hi there,
Please can someone help me with my believed Fiat UNO.
For the last few weeks it has had an intermittant problem. It starts fine and then drives well for a few miles and then becomes, what I call "lumpy". It then slowly comes to a standstill. I wait a couple of minutes, it starts again, no problem. It drives well for a couple of miles and then, the same thing again.
I took it to a trusty garage who informed me there was a fault with the alternator (electrical strip) which they replaced. A few days later- same thing.
I took it back and was told "Timing" which was fixed. For a few weeks fine, but now the problem has returned.
Any ideas please
Thanks you so much
Simon UK