First off, I think we can be safe in concluding your furnace is over size
second, you say the returns are two--14 x 8 and a 6" ???
a 120K BTU furnace will move 2000 cfm. On a furnace this big, I would install a MINIMUM of 10 x 24 in a bottom return or two 8 x 24, one on each side of the furnace. I think your return is inadequate.
NOW.....not all is lost. The TUY120R9V5V can handle a 3½ ton air conditioner. Increase your return air to, two 24 x 8 (24 x 10, if it will fit) and--->
check your dip switch's, I would set them up this way for 3½ tons of a/c~~~~~
1st stage heat, DIP SWITCH 7 & 8 to NORMAL. sw7---on, sw8---off. with a static pressure of 0.5 this should give you a fan speed of 1370 cfm with a delta T of 49° (for *HI* speed set them to sw7---off and sw8---off)