DIYguy,
Jeff has pretty much nailed the X10 response to Insteon commands so I'll focus on the CM15a. My Testerlinc does regularly produce BCY and BSC codes during CM15a Extended Code transmissions. I get an occasional BCY code during standard X10 transmissions. The CM15a transmits extended code messages for LM14a 2-way lamp modules. If you are using newer X10 switches and lamp modules with the soft start feature, you should be using the LM14a module interface within AHP to alleviate possible problems. The problem here is that your Insteon units will not understand the Extended code transmissions. Smarthome never implemented the revised X10 protocol which includes the Extended code direct dim commands.
I have seen a number of people report the BCY/BSC codes with the CM15a over the years and have never seen a satisfactory explanation. The CM15a may be shorting the 3-cycle gap that is specified by the X10 protocol. During extended code transmissions, the testerlinc may be getting confused by a protocol that really isn't in it's repertoire. Regardless, all of my X10 units respond correctly and both my ELK-ESM1 and the Testerlinc show 0 line noise.
My point in all of this was that the BCY/BSC codes may be normal depending on the type of communication you are using with the CM15a. If the codes are present only during valid X10 transmissions, and all of your units function, I would write them off as an anomaly. If the BCY/BSC codes are continuous you either have a looping X10 command (stuck repeater loop) or noise that is being interpreted as BCY/BSC codes. Your Monterey has a noise mode that presents a measurment 120Khz noise present around the zero crossing of the AC line. What is it telling you?
Regarding your Boosterlinc enabled Toggleinc, I would certainly try disabling the Boost feature in the switch. I do not have direct experience with these devices since I replaced all of my older boosterlinc devices with Insteon some years ago (didn't use Insteon with the Boosterlinc switches).
Boiler