09 Fiesta speed drops when weather bad

[Janey30381]

My son has a 09 fiesta 1.25 first car.

When we had bad weather (snow and ice) few months ago the car only drove  a mile up the road an the speed dropped and cut out.  We managed to tow it to local garage who ordered him a new set of spark plugs and leads.  Its been fine since the but when we had bad rain an thunder yesterday it done the same after a 10 min it started up an its fine now.  Henhas taken it to thr garage next to work and they have connected it to the computer and its coming up with no faults.  Am worrying about it as he starts a new job Monday and has to go on the motorway dont want it happening on there.  Any ideas what it could be? Thanks 

[Luke4efc]

Suppose you could check the spark plug wells to see if there's any water sitting there.