I have a bass that won't go out of tune either...It was a real PITA too--Back when I was playin' in the road bands our guitar boy quit tuning it between sets...He got real lazy, no cleaning, buffing, cradling, carressing...The axe developed an anxiety and would only play Elvis Costello shit for the longest time....But it got better...
...I just checked--Yep, still in tune...
Eric