Intermittent Serial1 communication with LewanSoul Bus Servo controller

I am having trouble consistently communicating with the LewanSoul LX-16A bus servos, using their servo controller, connected to Serial1 on an Arduino Mega. Testing the same read and write commands, I can often successfully read and write them, but not consistently -- making me think it is a problem with the serial connections (RX/TX). After an unsuccessful write to a servo, if I send the command again, to the same servo, sometimes it will work. I have tried the following without success: - Adding internal Arduino pullup resistor to RX - Adding external pullup resistor to TX - Adding small delays (0.1 millisec) between commands - Suspending communications with Serial (monitor) while writing the servos My code is rather elaborate, but I experience the same intermittent problems using LewanSoul's published test code, making me think it's related to the serial communications. (I have also tested individual servos and the LewanSoul servo controller using their test software to make sure I don't have a broken servo.) I am powering the Arduino through my USB port (it's receiving 4.9 volts) and the Servo controller has a 7.5 volt power supply (for the servos) which it regulates down for the controller electronics. (The grounds are connected.)

Thanks for any advice or help! -AJ

I suggest getting an O-Scope or an inexpensive Logic Analyzer so that you can prove that the data is not being sent.