Dell 1546 not starting

I have a Dell Inspiron 1546 that I bought about 4 or 5 years ago. About a year ago I would have to push the power button several times for it to boot up. It got progressively worst until about 6 months ago it would not boot up ay all. I purchased a new laptop and put the Dell away. A month ago I got it in and it powered right up. I used it for two weeks for a charity that I am involved with until it again quit, blank screen. I spoke to some people and the consensus was the motherboard failed. I purchased a refurbished (by Dell) motherboard and installed it. Now the computer boots up, fan starts, runs about 15-20 seconds and shuts itself off. I tried switching the Ram (2 X 2) and also removing 1 at a time and powering up, all with the same results. I removed the battery and tried with the power cord plugged in, no luck. It got very hot before it quit the last time and I have since cleaned the heat sink and fan. It won't stay on long enough to try anything with the software. Any suggestions?