1985 Volvo 240 Overdrive problem

I have a 1985 Volvo 240 with a manual transmission w/ push-button
overdrive. The overdrive has not been engaging and here is what the
car will do when attempting to shift into overdrive:

Driving in 4th gear, I press clutch and push overdrive button

Hear relay click, light goes on, release clutch

Overdrive does not engage

If I let off the gas, the RPMs drop to about idle and I can feel the
tranny trying to grab a gear as the car makes a jerking action.

If I get on the gas again, the RPMs resume what they would normally be
in 4th gear, but overdrive has not engaged.

I have checked under the car at the solenoid to listen for a clicking
sound when OD button is pushed w/ key on, motor off and I can hear the
click.

Any help on this issue would be greatly appreciated.

PS...once in a blue moon, the OD will actually engage but after about
ten minutes it disengages and will not re-engage.

 

http://www.swedishbricks.net/faq/transod.html#jumping

 

As the FAQ someone else linked implies, check the oil level
first. If that's ok, the O/D solenoid probably needs to be replaced.

 

And if that doesn't work, the piston seals in the overdrive are probably
shot, or something major is broken.

 

After low tranny fluid, since you hear the cli9ck of the solenoid, time to
replace the actuating piston seals.

http://www.brickboard.com/FAQ/700-900/OverdriveRebuild.htm

Duane