P0300 is a Radnom/multiple misfire...
Maybe a CKP sensor intermittantly faulty? Bad coil, what kind of spark plugs, wires and coil were put in. CMP sensor could also intermittantly be faulty as well.
As for being mad about having to pay twice for diagnosis, that is fair. Your paying for a licened tech. to spend his time looking at your vehicle. Maybe a reduced rate would be more understandable but free, hah. Do you go to your dentist and get a cavity filled and go back 1 month later and demand he fill another cavity for free cause you were there a month earlier for a previous cavity...?
1999 Jeep Cherokee SE l 4.0 HO l AW4 l NP231
Dana 30/Chrysler 8.25 l 31x10.5x15 Wrangler MT/R
Flowmaster Delta 40 w/stock pipes
Optima Yellow Top l Custom Headlight Harness