Could be a whole bunch of things. Start with simple stuff first - basic tune up, like plugs, wires, cap + rotor, fuel filter - make sure all is up to snuff. Maybe it's time to clean the throttle body. Run a bottle of Techron or BG44K thru the fuel system.

I think your '85 has the K-jetronic injection, and I'm not real familiar with it. But - does the idle increase to normal, once the engine is warmed up? If so, then you need to figure out what cold enrichment/idle scheme is not working. Perhpas a coolant temp sensor has gone bad or has a bad wire. Does the engine start OK? If it's hard to start, then maybe you have fuel delivery problems like a weak pump, a bad check valve, or even a marginal fuel pump relay. Just a few general ideas to ponder.