Will only Jump start
 

I have a 2000 Blazer LT 4WD and in Nov. 2009 I came out on coool morning and the blazer would not crank over my neighbor gave me a jump and it started right up, so having the car for 3 years I figured the battery was bad so I went and replaced the battery and the Blazer has started ever since not a strong start but it would fire up usally first crank, since Nov. I have drove on many 300 mile trips and have run the Blazer back and forth to work with no problems and my gas milage has been 16/23. I went to leave the store the other night and the Blazer would crank hard but would not kick over, I had it towed to my house and a friend of mind suggested we try and jump it, sure enough after 5 min. of charging she crank right over, I drove it for 8-10 miles shut it down and and cranked it up 1 hour later no problem, I went ahead and charged the battery thru the night and the next day put it in and it cranked right over very strong, went to work and left a few times for errands and when I went to leave at 5pm the Blazer would not crank over I had a friend jump me and it fired right up, I drove to the auto supplie shop were I purchased the battery and they checked the battery and alternator and both checked out fine, the battery was reading 13.77 volts so I started the Blazer up and drove home that was on Friday evening, Sunday I got in and tried to start it and again would crank but not start so I brought the Battery charger out and hooked it up for 5min I did not disconnect the Battery and low and behold it started!! Monday same thing for work but once at work it would not start to go home but would jump off I went and swapped out my battery it was reading 12.77 volts so the parts supply just gave me a new one, went back to work placed it the Blazer and would crank hard but would not start so I jumped it and no problems, any ideas from any of you would be helpful I am trying to avoided the repair shop after reading many articles it appears all the repairmen go for the fuel pump or alternator with only short term sucess it seems my battery has enough juice for most operations but just not enough for starting?????????? Please send suggestions

Fixed
 

It was the fuel pump I was able to jump start because it was giving my week fuel pump extra voltage, my mechanic claims I still had the orginal fuel filter my blazer has 174K so not to bad but I will make sure to change more often I also changed my fuel relay for peace of mind.