Why does my car drop speed then pick up it up again?

Yesterday i was driving a 15 mile journey, i came off the motorway and was driving on an a road when the car speed just dropped, my foot was all they way to the floor and still the cars speed just dropped, it got to about 15mph and then suddenly picked up speed again and drove normally, a couple of minutes later the exact same thing happened, there was nowhere for me to pull over so i had to keep going, i exited a roundabout and again the car would not pick up speed, the engine light started flashing and then the car started making a choking noise, it kind of sounded light it was coming from the back of the car like exhaust maybe.

i was going to a friends birthday party so left the car there for about 5 hours parked up, when i drove home, the car drove absolutely fine apart from at 1 point for a couple of second i could not get her car to go past 50mph.

the car is a ford ka 2003, done about 74000 miles, i got it last month but before that the car had been off road for 2 years.

this is the second time this has happened now, the guy that sold me the car also fixes them and he said before it was a blockage in one of the pipes, he replaced it, this was only on sat, the car drove perfectly then and did Sunday morning until i did the last journey what could it be?

i don't know much about cars so please keep any explanations idiot proof… Thanks!

The only thing you can do is take it to a decent garage and get a diagnostic check done on it which will show why the warning light has come on and may throw some light on what the problem is.
There's no DIY solution and yes, it's going to cost.

Could be a number of things. Sounds like a plugged cat converter. Misfiring of cylinders maybe Maybe a fuel pump on the way out. The engine light going off sounds like an expensive repair on its way though

Clogged fuel filter. Yes, it was changed already but as you drive with fresh fuel the tank is probably releasing the crud built up from being parked for two years.