My guess is your compressor is surging. Surging is caused by excessive pressure due to overcharging and/or air in the system. Remember, R134A has a higher volume than R12 in the vapor state. This means that if your system was charged with 100% of R12 capacity, it is overcharged. 80% of R12 capacity should be the proper charge. Your cooling effect may not be affected if the overcharging is not severe.