New 944 Wont Start

Just got an 85 944.  Was sitting in some guys yard and he finally got it running and sold it to me.  Ran fine for a few days and now it just wont start.  If I leave it sit a while, it will almost start up again but then after the first attempt it becomes useless to try. Any tips or suggestions?

check all the leads to the battery. check the age, erformance of the battery (you might need a new battery)

if this fails then it might be the starter motor

I tried all that, I actually went and bought a new battery just to have a new one anyway. New spark plugs and the starter sounds alright.  I just went to the guys house that I bought it from and asked what he thought it may be. He said that it ran out of gas right before he gave it to me and he put just about 1 gallon of gas in and "to be safe" a full bottle of fuel injector cleaner stuff.  Could this have caused a problem? I called a local autoparts store and asked them and they just said tap on the fuel pump and maybe its just clogged up but still no luck.

(stupid question but...) you have filled it up with good quality fuel haven't you ?

mine always started spluttering when the tank got below a quarter full... might be getting the tank cleaned out.

Ya I filled it about 3/4 full and still no luck.  I guess that it could be that the small amount of gas the guy put in and the large amount of fuel injector cleaner just havent gotten through the system and could be the culprit i guess.  PorscheDoc on my other post said try testing the air sensor and it's good, and to check the DME relay but i havent had time to get my big hands in there and grab it out yet.

I did not say check the air sensor, I said check the speed and reference sensors.  

ya sorry i meant speed and reference sensors, i was surfin the web for other possiblities and i came across somewhere that said air whatever and it just was stuck in my head and accidently typed it.

What were the values of the speed and reference sensors?

Go back to the basics.

Do you have fuel pressure at the rail?  What is the value?
Do you have spark at the coil?
Do you have spark at the plugs?

Alright, I personally didn't check the sensors, a buddy of mine who is a mechanic had it towed there and he said that it was alright. And I did check for sparks and those were all good.  Once it got towed back here I tried to start it and it actually started but was very weak and I had to keep my foot on the gas slightly to keep from dying.  A few more tries and it almost seemed to run normal once, but when I tried it again a little later it was weak again.  And the man that I purchased the car from said that for the fuel rail, he "rebuilt it and put in new O-rings". So maybe he just messed something up a little bit?

There are no rubber seals in the fuel rail, but rather on the injectors themselves.  There is really no way to screw that up to impede with fuel flow.  It would leak before anything. 

What is your fuel pressure at the rail?

Is your tach bouncing when you crank the car (slight movement)?

sorry i havent been on lately to post.  But all is well, well not all.  But the porsche starts and runs.  Although now i cannot keep a charge and it has to be jumped everytime i want to drive it.  And to top it off, whenever i put it into reverse the whole back end starts shaking madly and it will only back up if i take it very slowly. What do you think that could be?