DownForce it's most likely your 3-in-1. Sometimes they go out of adjustment for no apparent reason. I've heard that on cheaper electronics sometimes the values on the trim pots creep.

If I were you I'd start by adjusting your TR trim. Try and decrease it a little at a time then test the heli and see if it helped. If memory serves to decrease TR trim turn the trim pot clockwise. If you've found that the pulsing stopped, but now the tail has lost some authority you may also have to adjust Left/Right Bias pot. It's a pain in the butt. Three trim pots to dial in one tail is a bit ridiculous. Anyway if you can't get the tail dialed in your 3-in-1 may be faulty. This happened to me, but Hobbico sent me a new one under warranty. Hope this helps. Good luck!