1998 Honda Accord irratic RPM

According to Haynes there could be one of a number of problems causing erratic or loping idle:

vaccum leakage
EGR Valve defective
air filter clogged
fuel pump not delivering sufficient fuel to the injectors
fule pulsation damper faulty
leaking head gasket
timing belt and or pulleys worn
camshaft lobes worn
problem in engine mount control system.

I would check out the first three and the leaking head gasket. In a car of your miileage I wouldn't expect to see timing belt pulleys or cam lobes worn.

Hope this helps :)