Boxster boggs & surges under 4k

[mfinity]

My '99 boxster under 4k in warm weather (> 70 f) surges and boggs. If the temperature is cool outside the problem is not noticeable. Could it be fuel mixture?

Accelerating in 1st & 2nd there's a noticeable acceleration and bogging spots between 1.5 and 4k rpm. It's really noticeable when it's really hot out. Rather than a linear power acceleration, there's areas in the RPM range when it feels like it looses power only to have drastically more power at 500 rpm faster then another 500 rpm it has a low power zone.

I know this isn't much to go on, but not sure how to explain it better.

Any ideas? Thanks!