00 box s rpms drop @4-3 then climb?

[mn box s]

I have recently had my car in for 60k service it has 55k currently the shop said it was a solid car. I purchased it about 3 weeks ago about a week after it was in for service the cars rpm's drop when under moderate to heavy acceleration. if you start @ 2k give it throttle it climbs near 4k and then hesitates rpms fall to near 3k then climb past 4k and may hesitate around 5.5k but not as drasticaly as 3-4k. I had the shop do trany flush and fill, also this is a 6 speed manual, it happens nomatter if a/c on or off and if traction control is on or off doesnt seem to matter eather. Please help. If the clutch was slipping shouldnt the rpms keep climbing and the car hesitate. Not rpms drop? any info would be good thanks.