So my kid was coming back from prom and threw a P1870 code. Scanned it with hp tuners and car drove home fine ish. I noticed during cruise at 70 mph in 4th gear the ROMs would go from 2150 to 2400-2500 sporadically. Fluids were flushed two days ago from the dealership so it’s not a fluid level issue. Could this be from the converter clutch slipping or a bad lockup? Car seems to shift fine. Light did not come back on after I reset it and drove 70+ miles home.

EDIT
98 A4
yank SS 3600
3.42 diff

 

First is good to know what model year...if the DTC comes back I'd take it back to that dealership...if you want to diagnose this DTC you can follow the flow chart below.