I don't know why they would adjust the blower speed all the way down to 975 on a 3.5ton system that is too low as far as I know...
The average cfm per ton is 400cfm per ton.. But now they say it is acceptable for 350cfm per ton so I would not set your blower lower than that.. You have a 3.5ton which would be 3.5 x 350= 1225 cfm needed for your unit to operate correctly..
Did they take any return or supply statics to see what the static pressure was?? This number would indicate if you have a problem with your ductwork..