Any body familiar with OLD GP's

[Andy943100]

My friend has a 1977 Grand Prix SJ- Hurst Olds edition. It is the 400cc motor with the 400tranny. I think the drive line has about 120k on and still can run amazingly well.

He is having a problem with the modulator valve in the trasmision. It keeps going out, about every couple of months. This then makes the tranny stuck in 1st gear while in drive. After he puts a new one in, the car shifts perfectly fine until it goes out again.

He is getting lots of mixed reasons for this, all the way from the wrong vacuum hose to extra debris in the tranny to just getting into a bad batch of valves.

This has stumped the two of us. Anybody have any info? or good places to look? it is appreciated.

Thanks
Andy