[Ramblin Fever]

How old's your battery? Have you double checked all ground connections?

Unfortunately, there is NO guarantee that what we'll suggest will work - but what I CAN say AND have experienced myself, is although a remanufactored alternator will test fine on the table, it doesn't always work under load on an ISUZU. I emphasise Isuzu because these trucks are voltage happy and if they sense a weakness they will act weird.

I went through 3 Checkers alternators before I gave up and bought a brand-new Bosch ($210), bingo - problem solved. However, my transmission was still shifting ok, it was just other issues, i.e. no radio, no headlights, no a/c - BUT a bad alternator CAN and DOES effect the transmission as well.

Basically what I'm saying is I don't know if a new alternator is your answer, but I can't say it isn't either - luck of the draw.

Transmission fluid level good?

Ok, you're saying that at times you can get in and drive for a bit and it's perfect, all gears? And then after it heats up it starts acting funny? Could also be a torque convertor issue - these typically act up once the transmission has reached normal temp.