Possible issues:

Distributor Cap
Distributor Rotor
Spark Plugs
Vaccum Leak
Clogged Air Filter
Fuel Filter
Carbon Build Up (search for "Seafoam" for fixes)
Throttle Cable Needs To Be Adjusted

There are many other things that can cause this, but that's where I would start. You 90K maintenence will include a number of those.