Transmission or clogged converter?

[89 Caprice]

2011 journey with 2.4l engine. It burns a tremendous amount of oil (4-5L/month ) , was told stuck oil rings. Burning this amount of oil for 5 mths. I have recently noticed it having poor throttle response(sluggish). There is new air filter and plugs recently installed also. On take off it is slow at gaining speed to the point where the motor is at high rpm and wont shift into second until the throttle is eased a little, then shifts and works good. When kicked into passing gear same thing (big roar). If you ease into the throttle it work perfect? Engine light did come on once and went back out during passing a vehicle but it was revving pretty high. I put it on a reader and a code 0300(multiple misfire) was present. Did a computer reset and tried it again, this time no code. Engine works perfect (starting, sound, etc.) other than oil. Where should I start, still owe for two more years. I can get the insides taken out of the converter but would the sensors work properly? Could it be the converter or tranny?