I was flying QualityWing 757, and I decided to mess with the FMC after 3 hours of unsuccessful attempt at FMC with PMDG 747.Apparently my flight plan is already loaded into FMC, and I picked BAYPO1 from runway 36L for SID from Tampa airport, and activated it.The plane flew North to BAYPO, then turned to fly to the route according to the flight plan. Then I don't know what happened but the planejust started turning and turning and drew a couple big circles between the 2nd and 3rd leg of the route. I had to stop LNAV and switch backto GPS + approach hold + altitude + speed hold in order to continue the flight. Does anyone know what might've caused the FMC to jack up? I'm been having tons of problems with FMCs, it seems so difficult to useeven after 3 hours of video + PDF tutorials...