Brake lights out at separate times, but both bulbs okay.

I would pull the rear brake light bulbs and top 3rd brake light wire first to check for a shorted bulb.Do you have trailer wiring?That can be a source of shorts too.Next would be a "short tester" be used on the brake light wiring.

Sounds like the brake lights are staying on, which is probably a faulty brake light switch on the pedal.

Open it up and you will probably find something loose at the bulb socket, either a wire or the socket itself may be bad. Try holding the bulb in firmly while someone hits the brake a couple times. Maybe you'll see the problem. That brake light is simply spliced into your other rear brake lights. So, unless the splice has worked partially loose, the problem should be at the high mount light there.

Since the high mounted brake light works you know that the brake light switch is working. Now you need to check the brake light bulbs in the rear tail light assemblies. The brake lights are separate from the tail light (parking or running lights). There are two screws that hold the rear light assemblies in place.

revove the plastic nuts, doesn't hurt to try,

two fuses one for each side if fronts work fuses ok ,check bulbs again across a battery ,and make sure the bulbs are the correct fittings ,ie-single pole or double pole

I had this problem on my 98. Turned out to be the brake switch which is located at the top of the brake pedal arm. I went to autozone and bought the switch (under $20) and the sales clerk actually put it in for me after I asked a couple of questions about the location of the switch ... easy solution for me (-;

The same thing happened to my 2002 Montana minus the power window problem.

The brake lights and the cruise control are somehow tied in together.

I went to the scrapyard and ripped off a couple circuit boards from an old Lumina ($25 for the set) since there were no Montanas on site.

The problem seems to be when one brakelight goes the bulk of the current transfers to the other light and will blow it too.

Anyways you said you tried replacing the circuit boards and it did not work so sorry I could not be of more help.

put some dot 3 in the brake resavor until it gets to the full line that may halp

Replace the brake switch located on the brake pedal shaft.