The next thing to do is run it on DC. It will do that unless you have disabled DC running in the chip.
If it runs well on DC then the problem may be the DCC chip itself or the DCC socket.
If it doesn't run well on DC with the chip still in the loco replace the chip with a blanking plug & test again.
You can test the chip by trying it in another loco that you know works well with DCC.
You can test the DCC socket by replacing the DCC chip with a chip that you know works.