[bstark]

Whew. You're at least in the ball-park now and with all of those new parts you should be within spitt'n distance of getting it working reliably!

Having the temperature vary by 5 degrees or so is, in my opinion only, normal and I would think that you could now moderate your system to allow for setting your control at the middle setting of 3 by experimenting with the placement of that new thermistor on the cooling fin by sliding it downwards by 1/2" increments at a time until your cabinet begins to warm up and then you can advance your control setting to a higher setting to compensate which would then give you some tolerance either side to accommodate for outside ambient temps.

Patience has indeed been your "haulmark" in this debacle!