I have aboout 10 X-10 devices in my home, and all were working just fine until I installed 2 Switchlink switches. They would respond to commands only some of the time, and other (1-way) devices stopped responding entirely.

I then installed a Leviton 6201 phase-coupler/repeater. This solved the problem where certain 1-way devices would not respond, but the 2-way switches seem to have a mind of thier own. Sometimes they work flawlessly, then later in the day, they do not respond at all. I have replaced the switches, tried different house codes, and turned off suspect things in the house (refrig / ac / fans, etc.). Still no reliable answers.....

Short of spending more cash on a "whole house noise filter", does anyone have any hints as to what I should try next ?

The winning answer will be rewarded with a six-pack of cold beer.

Thanks.

Sounds like a noise and/or weak signal problem.

If your signal was marginal before the addition of two 2-way switches could turn marginal into not working.

A 2-way receiver has the effect of slightly attinuating X10 signals of the powerline. This normally isn't a problem unless you have a lot of 2-way receivers but if you have existing noise problems the additional attinuation might make the difference between reliable and unreliable operation.

Try the usual strategies for dealing with noise and see if that improves the situation.