[Healeys] trans issues

[BEAU2EVE at aol.com]

Hi guys, another issue has popped up this time a trans problem. When I  put 
the switch down to go to overdrive it goes right in smoothly an will stay 
in  as long as I don't give it a lot of gas,the harder I press the pedal it 
will  jump out, going back to 4 and stay there until I back off the gas, I 
just  changed my oil to a straight 30 ND. Owned the car 32 years never a 
problem,  what's the matter with these cars can't they make them to last? 
Seriously, does  this sound like a pressure prob or just solenoid arm  alignment.   
 Beau