No reason lag issue :(

Me and my friend were playing in his server for the last 2 months , without lag. We stopped playing for a while , and now , I'm the only person of the server with a unstable ping, (most of the time 350 ms). In any other server, my ping stays stable. The same for others games.I do not see any problems with my internet when I log into my friend's server. I don't know what is the problem with the lag issue , and I need help