Here's a tip if your XP stops counting and racing a different player or disconnecting doesn't fix the problem:
"Player1: create an unranked 'Custom Match';
for expediency sake select 'Pursuit Tag';
for expediency sake set 'Max Players' to 2;
Player1: invite Player2 (anyone who also wants to start getting OXP again);
Player1: start the race;
Player1: 'exit' once the race starts (i.e. disconnect), which will end the race;
Player1: Join Session in Progress by selecting Player2 from FL/Recent Players;
Player2: start the race (after Player1 returns to race lobby);
Player2: 'exit' once the starts, which will end the race;
both players resume normal racing habits until OXP stops going up again;
rinse and repeat as needed. "
Credit goes to xOBKx on the old Xbox Forums
Another thing I found out on my own:
If your XP stops counting and you keep on winning races all the XP earned during this "XP glitch" will count again once you solve the "XP glitch" (by the method described above for example).