Weak spark usually rears its ugly head under acceleration, more voltage is needed with higher cylinder pressure.
It could be dirty fuel injectors; pour a bottle of Chevron Techron in the gas tank.
It could be a small vacuum leak.
Does this car have ignition wires? You could have damaged an old, frail wire when you changed the plugs. Should be changed anyway if they’re 15 years old.
Next step after all that would be a compression test.