Thanks to the generous help from several members of the 070 and linuxham groups, my problem seems to have been solved!
Paul W7PBF suggested additional ferrite. Based on his advice, this article was found on the DX Engineering website, specifically addressing the RF suseptibility of the Signalink USB:
The article suggests adding a split core ferrite to the USB input cable, with 4 turns through the core. Don't have a core large enough to get 4 turns through it, but took one off the power cable of my computer monitor, and was able to get 3 turns through the core. Also left the small core that was previously there on the input cable. That seemed to have worked! Just spent the past 15 minutes qrming the 40 meter band, running 15 short test transmissions. Each time, the rig was keyed and the power meter showed output power.
Ed W3NR and others suggested that my DLY setting was too high, so it has now been reset to 0, and will be left there. Thanks for the feedback.
Lev HA5OGL suggested some adjustments if using VOX. Lev, when using the Signalink the VOX circuit is turned off on the rig. Thank you for your suggestion.
David K9DWR suggested building Fldigi from source, and also suggested contacting the linuxham group. Both great suggestions! I may still build Fldigi from source, to get a more up to date version. Also may try to build Flrig. David also asked about the CAT control device being used. I use the RTSystems USB-63 cable, which works great for rig control. The cable is simple to set up, and works well with Flrig on Debian.
Thanks to everyone for your generous help. I am grateful to everyone for your support, and will be heading over to the Palomar Engineers website to purchase more ferrite!
With best regards,