Freezer freezes but defrosts to much

Question

My Freezer gets down to 15f but then after a while shuts off and starts to defrost things in it. I have opened the panel and there is no frost buildup on coils. Is it the compressor or thermostat, defrost thermostat? Had AC man come out and he really didn't check much. Said it may be compressor issue. If so why does it get below freezing.