02 stalling

hello I have ad an issue for a few weeks but it seems to be getting worst...in the morning ill start my rodeo no problem I shift into reverse and rpm drops all the way down almost to the point of stalling.....then I seems to cacth back up to it self then run ok I can back up shift to drive and drive off with no problem..the last 2 days same problem but now starting while I drive off in the morning maybe 1st 1000 feet rpms will up and down almost turning off till it starts running better....thanks for any advice