I'm thinking the old motor was a 4 sp and the new is a 3sp but the HIGH Should be the same speed if they are both 1075RPM correct?
Right on this. Wonder if the run cap was replaced? (usually clamped to the outside of the blower housing)
You can pick up a run capacitor for a few dollars and hook it up, just two wires.
Like Wuzz was saying you may have a fan relay on your board could be bad. In my experience, fan relays tend to be chincy on alot of these control boards.
But even if you have decent air flow, you shouldn't be freezing up. I'd have the charge checked or check it yourself if you can. You can buy your own guages for less than the price of a service call. If you have 60-70 psi on the suction side you're ok.