Bicycle Tutor

So I have an odd problem,that leaves me helpless. I welcome any ideas. I just purchased a new(to me), but slightly used, MT bike. Cannondale Adventure 400. The bike has a Shimano Nexave front derailer over 3 chainrings. And a Shimanno Deore rear derailer operating an 8 cog cassette. My shifters are SRAM MRX PRO 1-8 on the right and 1-3 on the left. This bike is basically brad new but was stored indoors for several years.

Here's my issue. The bike shifts absolutely perfectly and quietly,however no matter what adjustment I have made, using the right shifter, I can easily shift to any gear,and it goes smoothly,yet when I release my had from the shifter, it wont stay put in that gear. It simply just returns to the smallest cog,even registering so on the shifter. is this an adjustment in the shifter itself?or is this just a broke shifter? Regardless I can use some assistance,as I currently use this bike as my only means of transportation,to and from work daily,down hill at a considerable slope for about 5 miles,but the return ride home,of course uphill,wearing slacks and a tie,and having to never let go of the shifter,is starting to annoy me, thank you all in advance for your help.

Either a broken, jammed, or gummed up shifter.
You can try getting some light weight oil/chain lube inside the shifter. Sometime the grease inside thickens up and will gum up the works. The thin lube sometimes dissolves the old grease and get's things moving.
But it could also just be broken...

Here are replacements:
rear only:
http://www.amazon.com/SRAM-Bicycle-Twist-Shifter-6-Speed/dp/B0017YRQKA/
set:
http://www.amazon.com/SRAM-Bicycle-Twist-Shifter-7-Speed/dp/B0017YX2OO/