Steve,
Been watching this thread for awhile and like seeing digital controls implemented in a simple device for making expresso. I am sure that some of the manufactures out there are watching your progress and like what they see. I have a couple of tid-bits that might help. Your control circuits common and the stainless/cast aluminum components of the brewer must be bonded. This will reduce and or eliminate noise issue. You found out that using the battery your unit was stable and then when you change to switching power source it would get noisy. I would try bonding the DC common of power source to the metal plate in the brewer. This will bond all the commons to the same rail. If any of the switches and thermos switches in the brewer are currently bonded to the plate on one side of the switch terminal, this would cause an issue and then I would not bond power source common to metal plate. Another thing I will bring up is that most ssr`s will switch comfortably between 20hz and 60hz. Anything higher than 60hz is a no go and will cause oscillation and or just poor control. Thermal controller such as a Omega control always offer 2 types of interface on the back. One for relays and one for ssr`s. When using ssr`s the control loop must work slower to work within the spec of the ssr. Also if your bonding everything together, make sure the thermocouple is not grounded.