I am new to this group but have been driving Jeep Cherokees and Grands for the last 10 years.  I am doing my first modifications on a 1999 Grand Cherokee WJ.  The only thing so far has been a K&N filter and I just had a friend machine a throtle body spacer.  When I installed the spacer and drove it, there seems to be a little lag on acceleration and then after it shifts from first to second it seems to rev for a split second, just a couple of hundred RPM.  It almost acts like the transmission is sliping, but didn't do it before I added the spacer.  Does anyone have any ideas?  Do I need to do something else?  Thanks in advance for your help.  

The computer in your WJ has to recalibrate itself to the changes in your intake.  When you do a mod to an ODBC computer controlled system it will take it a couple of days driving to recalibrate itself to the new condiions.  Drive it for a week and see if it gets more normal.

Thanks for the info, I will drive it for awhile and see if it starts running correct.  I did notice that it only does it under normal acceleration and not under hard acceleration.