1993 century trans problem

This car has 3.3l motor.when i take off in overdrive the car will run fine,untill i slow down to less than 5mph then i have no forward gears.when i come to a complete stop i can put it in reverse then back to drive it will go then.if i drive in regular drive i have no problems at all.this sounds simple to fix but im simple minded.have been told it sounds like the filter has come loose.please help if u can.

When was the last time the tranny was serviced....ATF and screen filter?