can anyone help please?
 

Hoping someone out there can explain why my Nissan Mistral ( terrano) is doing strange things, as the garage cant seem to help. It has an automatic gearbox, and has a switch for what i presume must be the something to do with the gearbox selection. this switch can be put onto power, hold or auto. I tend to drive with it on auto all the time, and it then automatically switches between power and hold ( which is much more sluggish) depending on how fast etc i accellerate. lately, i have found that sometimes when i accellerate, if this switch is in the auto position,the car wont change gear, and screeches along. I have to flick the switch to one of the other positions to make the gear box change. i then have to continue driving with it in this position otherwise it does it again next time i pull away. It has been suggested that i should change the automatic transmission fluid. has this happened to anyone else, or does anyone have more knowledge than my garage please? i have done approx 65k miles. your suggestions would be appreciated.