I have the same problem when I travel by air. My experience is that airplane seats don't breathe like other fabrics/seats due to their secondary use as floatation devices (I've had similar problems with sitting in leather chairs). One of the best remedies I've found to at least remediate the discomfort is to use talcum powder prior to boarding, and reapply during long flights.
Another potential cause is that the total time sitting during travel can be quite high. I find that I have less sweat issues when I refrain from sitting in the boarding area prior to boarding.