I don't think I would suspect either the thermostat or icing. Icing usually in cool humid conditions but under hood temps at 90º in Ca unlikely to be in that area. If you fitted this as a complete new kit it's known that the jetting is often slightly adrift for some cars. IPD used to, may still do, offer a jet kit so you could tweak the jetting. You paid a deposit and got your money back less the cost of the jets you kept. I would check the floats and needles for free movement. Floats might stick low as you go down hill giving very little gas when you come back on the throttle. You say it is an E engine, what distributer are you running? There may also be issues there.