Can you suggest a way to take care of this annoying issue?

Your missing the biggest factor - as hot air rises, guess what happens to cold air in the house? it settles in the basement. If you can keep the door to the basement closed, it will minimize this. Depending on how the house is constructed, you could also put a circulating fan to blow the cold air upstairs. sounds like you've done everything right to seal the A/C system. Maybe another cold air return into your system from the basement would help too that way your A/C system could pull the air from the basement upstairs. This would also save you money on your A/C bills as the compressor wouldn't have to run as much.