can anyone explain? my 126 each time it changes gears when driving (especially cold) it revs until it catches the next gear.

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99 C43
98 S420
99 C230K
01 C240

A slipping transmission can be one which revs freely in between (during the shift) gears.

126 transmissions usually do this on the 3-4 shift when they are on the way out.

Check your tranny fluid before anything else. Also check for leaking trans cooler lines.

Let us know.

Mike