I had tons of problems with the Robershaw performance stat, marketed under Robertshaw, Moroso, Mr. Gasket etc... I went through 6 stats to include the previously mentioned "balanced thermostat" before I gave up and replaced it with a run-of-the-mill Stant Super-Stat. The temps hold steady now as they should with no hint of any problems. Cooling doesn't seem to be inhibited either with the Stant.
A purposely blocked coolant bypass will generate heavy temperature swings too as it hinders coolant flow through the engine with a closed thermostat. You'll end up with steam pockets in the heads etc. Drilling one or two 1/8" holes in the t-stat WILL NOT provide you with as much flow as the bypass will, hence the temp swings too. I'd been down that road also.
Dave