Well I have been using the Garmin FR230 since October and have found it very reliable. No problems at all. I deliberately avoided the FR235 since there has been some debate about the general reliability of most brands of HRM on the wrist. If I want a HRM I can use a traditional chest strap with the FR230.
The cuboid can be found in front of the ankle. When seated reach down to the front of the ankle and you should find a little hollow space between the ankle and the cuboid where there is soft tissue of about an inch long. If that is the source of your pain then we have the same problem.
Keeping your foot on the floor lift the arch slightly so your foot is resting mainly on the outside edge. Your fingers will feel that hollow space open out. What causes the pain is that hollow space collapsing inwards when you run gripping the soft tissue and small ligaments.
The solution offered to me was orthotics to keep the arch raised while running. I have been running a long time and only occasionally have had this pain while training, thankfully never during an event. I never went for orthotics and just rested whenever affected. Possibly my gait has adjusted over the years to compensate.