Atmega328p - Program works with 2.7V power supply but doesnt with 3.3V
From section 8.2.2.1 of the datasheet, it is likely your regulator is unstable. You should have 10 \$\mu\$F on the input and minimum 10 \$\mu\$F on the output. When you feed the regulator 3V3 as an input it is not acting as a regulator; have a look at this question.