Things to check for Lambda Sensor error

I am intermitently getting a check engine 5-8 light while driving. Seems to happen on the highway after a spurt of speed, such as when passing someone. Light will come on temporarily and go out again. Code is 1114 (Lambda Sensor). I have reset the computer, added a ground wire to the O2 sensor, and even replaced the sensor.

Car is a 1990 Mondial T Cab. Using test pipes and Borla sport mufflers. Mufflers have been on the car a long time, but the test pipes are new. Not getting the error on bank 1-4.

At this point, I'm not sure what to check next. I don't sense any power loss when this happens. Any suggestions as to what to check next?

My friend's 94' 348 spider was doing the same thing (5-8 light when accelerating). His Nippondenso alternator gave up the ghost a couple months later...and this alternator had be rebuilt only a year ago. You might want to check your alternator output (esp. at higher rpms), and check and verify that you have a good ground between the alternator and chassis. Perhaps even adding an additional, thick ground cable between the alternator and engine.

Update..,my friend found the connector for the alternator very dirty and corroded. He has cleaned and treated it, and appears to be "good to go".

Thanks for the feedback. Sorry to not respond sooner, but my harddrive was eaten by windows so my computer access has been limited. Working with a borrowed laptop for now.