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T2.5 pulley with GT2 belt would do no good, but won't cause regular shift either.
Typically such shift is matter of undercurrent or too high movement speed. In both cases, the motor is not able to stay on microstepping and/or do the required amount of microsteps. But that is mostly the case of Nema17, not 28BYJ-48 due to the internal gearbox.
First, check if the pulley is correct for the belt you are using. Than try to print at half of the speed you are using now. If the problem still remains change the motor. Some teeth may be missing in the gearbox.