R51 -power problem

[R51owner]

Thinkapd R51 1836 -BAU ....... Recently started, and quickly escalated, problem with power. I usually run with A/C adapter on, and put machine in sleep mode between uses during the day. Lately it randomly decides to power itself down. Yesterday it only ran a few minutes and BEFORE I put it in Sleep mode it powered itself off. I must say all this was with the battery in, almost fully charged and A/C connected. So, I took the battery out and in my last few usages it has not powered itself down, only when I do power down command. I haven't tried to use sleep mode with no battery and only A/C power..... I haven't tried to run since this began using only battery and no A/C, either. So, should I run PC Doctor? ANd then what? Does it just need a new battery (almost 3 yrs. old and doesn't hold long charge)? Any others have similar problem and SOLUTION?