I have a 1991 ford ranger with the 3 liter and 5 speed. When I shift from 1st to 2nd the engine bogs down and eventually stalls, but if I push the clutch back in the engine goes back to idle. Any ideas?

But when you shift into first and let off the clutch it goes perfectly fine?

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2003 3.0L Flex 2WD
K&N drop in filter with Air box Mod.
235/75/15 bfg r t/a sports.

yup, when i put it in 1st and start going, its fine. but when i shift in to 2nd, it starts to stall.

Anyone? I'm thinking a problem with a sensor on the transmission?