Isuzu Rodeo Flashing Transmission Dash light and Clunks?

2001 Isuzu rodeo 4cyl automatic. This is very ODD. At times the transmission dash light will come on and the transmission will make a huge clunk noise. This is very inconsistent. Does anybody have any ideas what it could be or why this only happens sometimes? Took it to the mechanic but mechanic can't get it to it. I know I am not nuts and vehicle isn't possessed so what could it be? Thank you for your time should you answer.