The infamous "flare" is similar/same as a slip. The RPMs increase between upward gears. 2nd > 3rd seem to be the more common combination. Vac leaks seem to be the most often mentioned culprit. My '87 300TD had that symptom until I ran Trans-X and found/fixed a number of vacuum leaks. Not certain which was the cure.

Several others should be able to provide more technical explanations. To me it felt like the trans was slipping allowing the engine to rev before settling into the next gear.

__________________
Steve
'87 300TD - 132K - Soon 4-Sale
'84 300D Turbo - 122K - Driving
'77 VW Type II - 77K - Restored
'08 250EX Ninja
English Bulldog (Brier) - My best friend. Passed away 12/02/04 while in my arms.