How do you solve high humidity issues in a home?

I live in the mid-west and have a 2 story home built in the 1950's.  Over time, the home was remodled.  The remodle resulted in 1 AC/Furnace unit for the basement/main level and a 2nd AC/Furnance for the upstairs (the upstairs was previously attic space).  In the summer the humidity is frequently in the 70's (yuck!) on the top floor and low 60's on the main floor.  Sadly, portable dehumidifer does not seem to help and I am tired of the humidity (my ceilings have started to crack).  I have had my furnance / AC checked, nothing.  I have checked to see if the house is poorly insulated and that is a possibility.  Do you have any other suggestions for fixing this issue?  Heat exchange?  Whole house dehumidifer?  HELP!