It seems that, it all boils down to one thing..Plug in Mod is not that great at all..Friend of mine had one and don't get me wrong but most games that were copied one to one from image to CDR(No patching) worked flawlessly however some patched games did not work and we needed to use Name Brand CDR's..TDK or Sony..I tried silver bottom CDR media's(Easier on the PS laser) and it's 50/50 shot of working..It worked once in a while..Where Sony or TDk loaded 100% of the time.
My advice is get a Stealth Mod chip..Your machine is compatible with the Current Stealth Elite Mod chip(Ver.5 and the Final) will do the job..It should run you no more than $10.00 for the chip and $20.00 for the installation unless you got a Soldering skills(It's not that difficult to do compare to PS2).
Also be sure NOT to run applications while burning, because this costs buffer under run and chance of Roller coaster are high..
I hope this works out for ya..
Usually any machine that's modded with true modchip only time it won't load is cause roller coaster or Bad media but in your case it's the Plug in mod..This is from my experience. Let me know how it goes..