According to AMD Specs on your new 78003D it Maximum Operating Temperature is 89c. So if it is reaching 88c it is close to overheating and to start to throttle.
The APU processor's TDP rating is fairly high at 120 watts so you need a powerful CPU Cooler. I would suggest a 200 Watt rated CPU Cooler or higher for this APU processor.
Adding to Johnny English previous reply:
To eliminate poor air flow inside your PC remove the side cover and see if the Processor's temps are slightly lower. If it is that indicates poor air flow inside your PC case.
Post the CPU Cooler's Make & Model that you have installed. Make sure it is installed correctly and making good contact and everything is working like the Fan and/or Pump (if it is a AIO CPU Cooler).