Problem with 96 Voyager Blower Motor

I have a 96 Grand Voyager SE 6 cyl. 3.3l van. My problem is with the blower
motor. At first it would only work on high speed. I got a tip to check the
resistor. That fixed the problem of all the speeds not working, but now
the blower comes and goes. Usually when it works it will work for a while,
but if I continue to drive for a long period of time, you can hear the
power getting weaker and weaker until it stops. Once it does that it won't
work for a few days, then one day I will start the van and it will come
back on. Sometimes if i hit a bump it will come on again. Another thing I
have noticed when it is not working properly is that the A/C will still
come on while the blower switch is off. Anyone have any ideas???????