My car is a Civic 96 EX manual with 120,000 miles

Since about two months ago, my car has suffered from the problem of suddenly losing power while I am driving during the first 15 min from cold start. So basically when I start my car and drive it on the road, during about the first 15 min, there will be a moment where the car suddenly slows down as if no gas has been pushed into the engine. This symptom happens in gear 2, 3, 4, and 5. When I encounter this problem, I shift to a lower gear - because the car slows down - and the car becomes normal again. Such a symptom may happen once or twice during the first 15 min and after that, there's absolutely no problem. But if I use my car every day, this problem will disappear - I usually drive it once a week.

These are the part replacement or services done over the past year:
distributor, spark plug and wires, fuel filter, battery. I also had oil change last month and tried techron fuel system cleaner. CEL is never on.