98 pontiac Bonniville- series 2 3800 V-6 surging at cruise speed.

Have 98 series 2 3800 v-6 auto trans. Problem: starts, idles, accelerates great. But at 2000 RPM, when driving steady in top gear, engine drops RPM about 200 revs and quickly recovers. If you set the cruise at 45 MPH or so, this occurs, but if you move the trans selector back to get the RPM up to say 2800-3000 RPM in 2nd, engine runs smooth with no drop or surge in RPM. Have replace air intake temp sensor, now suspect idle control sensor may be causing this. Am i on the right track? Car accelerates from stop to redline at full throttle without missing a beat! Replaced spark plugs and all old suspicious vacuum lines. Very smooth idle. No hesitation even with sudden bursts of throttle. Any ideas?

Thanks,

Dave

Welcome to AF, this sounds like the lock up convertor is engaging and disengaging, many things can cause this, bad tps, bad tcc solenoid, internal trans problems, without a road test hard to diagnose from here.