Auto Parts: 98 chevy astro van, chevy astro van, chevy astro

Question
i have a pain in the ass 98 chevy astro van, 2wd. it was very reliable until a year ago. for some reason my truck will work for 4 days then not work for a couple, then works again and then wont. it has been doing this for the last year. i have taken it to get fixed 4 times. everytime i bring it in they either say i cant do anything while it works. so when it doesnt they just replace some sensor and then the trucks on and off cycle starts to work again, then the mechanic says its all set. i take it home then a day or so after it doesnt work again. i take it back to the mechanic and they say some bs and then tell me they dont know whats up, and basically to get the truck out of their shop. i dont even know where to begin. i try to start it up when its not working, the engine sounds strong like it wants to kick over but doesnt, and when i turn the key to stop it, the truck starts to shake like it wants to do something. the whole time the trucks engine/battery seem to be strong enough to start, i have replaced the battery and i get the same thing. when the truck does work it starts up no problem.. i did try to tow my truck the other day and the tow truck driver got it to start after about 8 mins of trying to start it over and over. i tried to do the same today but no luck, it did seem like 1 time it was going to start but didnt. im am so clueless and it seems so are the mechanics around me. one thing i do notice is, when i put the key in the ignition and turn it just enough to turn on my electic(radio,dashboard lights) if the battery reading goes to at least halfway then the truck starts. when the reading is at like 25 percent it wont. ihad a airflow sensor replaced as well as a map sensor and still same issues. i had the fuel pump replaced about 16 months ago. the fuel pump activates fine, so im not sure thats the problem. someone i trust said to try to replace the fuel filter, and check the fuel injection. aslo, i have not had a tune up since i guess the original one, i got the truck at 65000 miles, im now on 173000, could it just be a tune up? when i asked the mechanic he just laughed. could it be spark plugs. i have no clue about cars but it seems neither does my local mechanichs. i would appreciate any help. thanks

Answer
    Jeez Louise!  This truck hasn't had a tune-up since you bought it?  Don't even think about doing anything else until you take care of that.  I know that the distributor is hard to get at in this vehicle, but it's not impossible and it HAS to be done.  I would recommend, since access is so difficult, that you replace the entire distributor with a remanufactured unit, adding a good quality cap and rotor (please don't go to Pep boys or one of those other trash merchants).  Also new wires and spark plugs.  I would even replace the coil, then everything would be new.  Make sure that the timing is working properly (a good mechanic will do this with an OBDII scope/scanner; you have apparently not met a good mechanic yet, so look around for one).  You can't diagnose anything on a gasoline powered engine until the ignition tune-up is 100%.  If it still acts up, and it may, then the fuel injection unit in the manifold is bad and needs to be replaced.  They rarely last as long as yours has.  There are even kits available to convert your engine from central injection to sequential injection, and they are worth the extra money.  And do change that fuel filter and any other filters that haven't been taken care of lately.  It may seem that you shouldn't spend a lot of money on an old truck like this, but the V6 engine in this van will typically last over 300,000 miles without an overhaul, provided you change the oil regularly.  It is a much stronger engine than the V8 on which it is based.  Get these driveability problems resolved and you can drive it another 100,000 before you do anything to it again.