I have a 2000 Mitsubishi Montor Sport i had it almost 3yrs and just in this last year it started to act funny with my check engine light is on

Check air filter and clean throttle body replace fuel filter if have not done so in a while add Lucas Fuel Injector Cleaner to gas tank.

Another problem could very well be that the alternator that you put in simply has issues I had a similar problem and as long as I had a fully charged battery it would run pretty well but once the battery Ward hours every light combination the world would spit and sputter and eventually not drive at all but when I had the alternator tested while running in the car it would show that it was putting out 14 volts which is about perfect however the alternator even though it was new it was an after-market piece of junk that wouldn't work consistently that might be your problem and it's not terribly expensive 2/2 a good warrantied alternator put in good luck I wish you the best and wish you were nearby I would help you myself as I have had so many issues with the same vehicle I have spent more on Parts than I paid for it unfortunately but now I'm at the point of no return I just need to keep everything there's nothing left to fix I guess

Take vehicle to your nearest auto parts store like auto zone they will scan the computer for you for free and let you know why check engine light is coming on.

Lean your throttle body. Mine did that after changing the battery. Trust me. Sounds stupid but it works.