RPM jump at 2.3K???

[porschin']

I have a 1998 boxster 986 that is completely stock, 55k mileage. I have a problem when the car is cold at constant acceleration I get a jump in rpms at around 2,300 rpms. It revs normally and then once it hits 2300 rpm it almost drops too 2200 rpms and continues to go up. I don't know if this is a transmission or clutch issue. It is a automatic tiptronic and has been garage kept for quiet some time. I also have been having trouble between gears but when car is warmed up. From fourth to fifth gear (car is automatic) the engine revs up in between shifting. Like the accelerator sticks when the clutch engages trying to problem shoot this, other than that the car runs like a champ. If anyone has a idea as to what this is I would greatly appreciate any pointers. I am in Denver at high altitude, don't know if that would affect it but car came from Tucson, AZ two years ago when I bought it.