1994 GMC Truck engine low RPM

HI there
there the situation, I did a tune up 6 mth ago, I replaced cap, rotor, spark plugs (AC DELCO ), wires, fuel pump, fuel filter, air filter, also replaced intake gasket ( was leaking), it ran realy good all that time, 2 weeks ago it started to acting up when come to a stop, the engine rpm drop dramatically almost to a stall, but then picks up again up to normal?And it does it also when I back up? Could it be a faulty sensor? Or my catalitic converter is pluging up?

thanks