when engine gets hot revs seem to wander up and down.i have been told it may be the lamba sensor.is this plausible or any other reason?

 

Idle air control valve (IACV) is much more likely. Remove IACV and clean inlet screen and valve movement thoroughly. Clean throttle body bore and plate from entrance w/ old toothbrush and throttle body cleaner. Check that PCV is clean and and PCV hose is open.

Is Check Engine Light on? If yes, obtain codes and post them for help.

good luck