So in order of proximity, you have:
- Computer
- bluetooth host
- does the dropout behaviour change if the computer is raised to head height and off to one side? (to preserve line-of-sight to the clients)
- would a USB extension cable with USB bluetooth dongle help the connection by relocating the bluetooth host to back of the ATV?
- Internal combustion engine with electronic ignition
- are you running a resistor-type spark plug? (for less electriclal noise)
- does the dropout behaviour change with engine on/off?
- Human body
- in the line of sight
- does the dropout behaviour change with the human is removed from the line of sight?
- Reach RS2
- bluetooth client
- EM38
- bluetooth client
- is the metal chassis of the ATV between the computer and the EM38?
Can you download a Bluetooth app that shows the RSSI or some other signal quality/strength data for each client? (maybe even for throughput - to answer your other question)