J-type problems required reading: http://buckeyetriumphs.org/technical/JOD/

Beside the general accumulation of gunk, solenoids do wear out and get "funky" in the heat, especially as they generate a fair amount of heat themselves. The D-type is far more suceptable to this and the D-type also requires the solenoid to be absolutely properly adjusted for throw or they will burn out quickly.

Another possible place for trouble is a vaccum relay/switch that was installed on some systems (usually the D-type) that will make the o/d "kick-out" under heavy load (lots of vaccum). I tend to dis-connect these things when found.

The overdrive is NOT sturdy enough for shifting under hard acceleration in ANY gear. You should always "lift" slightly when engaging/dis-engaging. All there is in there is a cone-clutch to lock the planetary gears to the sun gear and it will wear swiftly if abused.

Yours engages in second??? It should only be engaging in third and fourth.

Mike!