99 Century misfire/overdrive hesitation?

Please excuse my length but I want to be thorough. 99 Buick Century 3.1 w/117,000 on odometer, I replaced engine at 90,000 with a boneyard engine that had 80,000. Tranny was rebuilt at 107,000, I had no problems until the last couple of weeks. I started to get a bad hesitation at each shift, did some investigating and found a loose vacuum line at the vacuum modulator. This fixed all the hesitations except from third into overdrive. The engine also misses, (oxygen sensor? code came up on the reader). My dad thinks the miss could be causing the TCC to engage and disengage causing the hesitation. Or possibly a problem with the TCC solenoid. Any help would be great and let me know if you have any questions. Thanks and this site is great.

Welsome to AF :wave:

Both issues (O2 and TCC) will cause driveability problems, additiionally the O2 fault code caould be a result of a TCC/VSS error.