Sounds like the governor valve is stuck, which is why it will shift out of first gear when you gun it. The governor valve is on the output shaft of the trans, you'll have to pull the tail shaft housing to get to it. If the Throttle pressure valve (Kickdown) was misadjusted, it would rapidly upshift to 3rd gear, unless held in gear manually. For the most part, a 727 and a 904 are almost twins. Once the TS housing is off, you will see the gear that drives the speedo-cable, and what looks like a set of boxy ears. On those ears there are 2 weighted flaps w/ light springs. As the output shaft spins centripetal force pulls them open increasing the pressure from the trans pump on the governor circuit, this will continue until the pressure is higher than the throttle (kickdown) side. At which point, the transmission will upshift into the next appropriate gear. No matter what it sounds like the problem is in the governor circuitry of it. Also, I have a 904 trans, from a Challenger that has been stored indoors, with the kickdown levers and such attached, if you want I will rebuild it, and sell it to you for a very fair price; or I can sell it as is.
Sent from my DROID RAZR using Tapatalk 2
