Overdrive issues

I hoping I can get some help in here. I used the search but couldn't find a solid answer. A friend of mine bought a 99 Taurus with the DOHC that seems to have a handful of problems. He only paid $500 bucks for it, but if we can get these problems worked out, it seems like it'll be a good car. The engine idles and runs really smooth. The tranny too w/ the exception of this issue. I'll list all the problems just in case some are related. I'm not sure if it's all just different problems or if some are causing others.

Here's the main problem right now. After driving for a while, the transmission O/D will start acting up. It'll drive fine, then all of the sudden it'll rev high, then slam into 2nd. The O/D light will blink constantly. From a stop, it will start off in 2nd or 3rd(not sure) and slowly accelerate. On the occasion it starts in 1st gear, it'll rev high, then slam into 2nd again. The O/D light blinks the whole time. It'll keep on driving this way until you stop the car and turn it off. Once you restart it, it'll drive fine for a while again. On a side note, the speedometer doesn't work either. The Service Engine Light is on so we pulled the codes but nothing was related to the trans.

Other than that, he's also getting codes...

P0171, P0174 and P1451

(I found the sticky for the first two codes.)

P1451 says Evap system canister vent solenoid circuit malfunction. Probably causes- failed canister vent solenoid, -Open or shorted circuit condition.

Like I mentioned, the car runs good at first. smooth idle, smooth shifts so I think if we get these things worked out, it'll be a pretty decent car for what he paid. He also has to...

-reinstall a stock radio unit(someone had an aftermarket and hacked up all the wires)
-replace the driver seat (broken seat back)
-fix the front passenger window assembly(probably needs the motor assembly)
-replace both lower control arms on the rear(not sure what happened here but they're bent. We already replaced a rear sway bar link)

Thanks for any help, sorry for the short novel.

If it's a dedicated transmission code, the common scan tool will not read it. Some transmission codes require a professional scan tool.

You may need to stop by the dealer or a transmission shop to get the transmission codes read.

My first guess would be an input or output shaft speed sensor issue. If one of the sensors is intermittant, it could be making the computer think the gear ratio is incorrect and cause the shift.

-Rod