code 51/ quits when returning to idle

[professorbob]

Still trying to get the supra back on line... Finally have an inside area with heat to do the work. Update. Car ran great for 120 miles then started cutting out and now will not operate above 2000. I have lost the gasket between the manifold and header I installed new when the engine went back in.
After some effort of multiple tries and seeming more fuel being added with each try the engine will start and seem to go through the normal warm up as required. I suspect that the difficult start is fouled plugs. As the normal operating temp is reached and rpm come back the engine goes through occasional hesitation. I can apply throttle until about 2300 rpm when it acts as though the ignition has been momentarily stopped until it goes back to idle speed. There is a very rich condition and soot is blowing out around the header. I have a replacement gasket ordered.
Can a clogged converter cause enough back pressure to cause a gasket blowout and these issues. The rear exhaust seem to be normal.
If so is there a way to determine converter condition without removing the exhaust again.
What about a bad O2 sensor as the one on the turbo housing. IF it is bad will that create these issues?
All suggestions are appreciated.

I am going to check codes, but expect that I have not been able to operate the engine long enough for the on board diagnostics OBD to make a check.