95 Prizm, can't get the shifter out of park

I have a 1995 Geo Prizm that has had very few problems in the 9 years I've had it. However, a year or so ago, after getting gas, I got in my car and started it but couldn't get it to go out of "park." A gentleman who was also getting gas came over and tried to help, but it wouldn't budge... Yet, after several minutes of working with it, the shift finally moved into drive.

This morning the same thing happened again and I had to depend on someone to pick me up for work. What could the problem be?

Thanks for any help you can give me...

First bet is the brake pedal interlock is messing up, does playing with the brake pedal or repeated stepping on it make a difference?

Second bet would be the ignition interlock is screwing up, if you can play with the key and the shift lever suddenly releases, that rules out the brake lock ;)

If you have a thumb button on the shifter, does the button appear to stick in any position? This is the 3rd possible cause, but rarely the problem. The brake interlock is the most common cause I've found on most Geo's over the years. (toyota and nissans as well)