My mitsubishi diamante will start up fine,but when u either put it in drive,or in reverse the car will rev up but not take off right away,then it will just pop and then take off.

starting the engine is one thing,going into drive and reverse is something else. Don't expect good answers from mechanics because there are all kinds of ways that trannys work. some are purely electric (2004 probably 100% electrical) but going back they were part electric and part mechanical governors &modulators, There are different symptoms,causes and cures depending on what tranny is involved.I was told by 4 different mechanics that nothing externally could be done with my own transmission but I knew enough to know they were all wrong. So I downloaded a complete ATSG manual ,read it,fixed my tranny.So get ready for voodoo and bs and big bucks OR TRY a little d.i.y..Start with basic fluid dump,filter and cleaning ( and you will be told that that will do nothing ;and,it may not - but it is worth a shot.)

Transmission silenoid going bad and need to change transmission strainer and fluids she should be good