trouble code 13 and 44--- 1985 vette

hello fellow 'vetters:
my 1985 vette is throwing code 13 for the oxygen sensor-I replaced it- and code 44, which is lean exhaust.  the car runs erratic-- for a while it runs great with full power across the full range of the throttle. Then, like a bat out of *** it starts running like somebody pulled the plug wires, BIG loss of power. After a few miles down the road, it starts running fine again. This pattern just keeps up! I have replaced the plugs with platinum, new plug wires, rotor and distributor cap. also, just had valve seals replaced as plugs were previously getting fouled up.  
Still , the car loses power intermittently and sets codes 13 and 44.
Any thoughts would be appreciated!  

So, you replaced the oxygen sensor, runs fine for a little bit, then runs like crap and throws the same codes again? So, you're frying oxygen sensors but not getting a trouble code for why.

First thing, check the wiring to the O2 sensors. With a car as old as yours, you might have a bad wire going to the O2 sensor.

If that's not it, then I'd look at parts that could be causing premature failure of the O2 sensor, which would be the Mass Air Flow sensor, Throttle Position Sensor, and the ignition module.

It sounds like something is causing the car to run pig rich or super lean, and it's frying the O2 sensor. How does the O2 look when you pull it out? If it's black and sooty, then the car's running super rich, if it's ash white/orangish borwn, then the car's runnign super lean and the excessive heat is frying the sensor.

One more thing to check, make sure your catalytic converter hasn't gone bad or is plugged up.