1998 s10 ss

New to the forum and I am hoping someone has some help for me. I purchased a 1998 s10 ss last summer that was in pretty good condition. I went through it with a mechanic friend and found it to be in pretty sound condition other than needing a tune up, it had a sloppy center link and a small leak in the lower intake gasket both of which were replaced. The truck ran like a champ but as winter approached and I stopped driving it other than on nice days I started to get intermittent error codes P0440 and P0300. I have had the truck in to the dealership and to my mechanic friend had the smog test on the evap system and they can't come up with anything. Any help would be appreciated. I should add that the tune up included new plugs wires cap, rotor and fuel system cleaning.

Ive been fighting error p0300 aswell..I have done crank pos sensor and cam pos sensor.. Look at the dizzy and cleaned the contacts same as rotor button. replaced MAF sensor. Timing is correct. spark is good compression is 160-170 lbs. fuel stays at 57PSI which is good. replaced coil.. I have heard I need to check the CAT converter.. we shall see..any ideas other than that??