Air in Fuel Lines.. PLEASE HELP!!

I have a 1999 Ford F250 7.3L, i bought this truck about a month ago and about 2 weeks in started having trouble. I fixed the truck by simply purging the fuel lines, but now air keeps getting in my fuel lines about every 3 days or if i run the truck hard. I've heard everything from it being the fuel pump to a pin hole in the fuel line. Any help on what you might think it is? Any help is appreciated.  
99' Ford 7.3L PowerStroke

-SuperChip
-K&N Air Intake
-Straight Piped

either a cloged return line from the back of the fuel bowl or a , your heads are messed up and u need new injectors and valves    i see this a lot with guys who have tryed bio-diesel  
97 F350, Stage ll hybrid injectors, custom 4 in exaust 6 in tip, 4.88 gears, custom turbo, custom  electronic work, zoom clutch, upgraded high pressure oil pump, cold air, 4x4 , 4-door, long bed 5 speed, HHO unit, 31 mpg, superchip, upgraded fuel galleys

i had the same problem it only happened when my truck went down to about quorter tank it was in the tank the fuel sending unit check ball was leaking air once it was exposed to air in the tank my truck ran fine with full tank ub untill i got down there it got air and to fix it i just cut the check valve out and put a longer hose on it to reach the bottom of the tank  that could be your problem  

Not to be redundant, but go through and check every fuel line before you do something really intensive like new injectors or a check valve in the fuel tank. Its the little things like a loose fuel filter or fitting that will get you. Can you see diesel pooling anywhere? I had an injector o-ring leaking on my 7.3 and the only way I was able to find it was with the engine running, I ran a dry shop towel over every fuel connection I could until it came back wet.
You say it only starts hard occasionally? This may sound silly, but I put about $800 worth of new fuel lines, injector pump, o-rings, and the works into my old 6.9, in and effort to fix an air leak. After all that it still started hard. In the end I just needed a new started because the old one wasn't spinning fast enough to start the truck.  

I haven't had any problems with bio diesel. I burn 7-10,000 gallons a year of B-20 on the farm and it has powered my '99 7.3 to over 200,000 miles on stock injectors. We have been burnning it for 10 years now with no troubles. Now for your air leak, check the fuel bowl cap. I had a ford dealer service my truck once, they changed the fuel filter and when they put the cap back on they cracked it. Before I left the lot I knew somthing was wrong. I went right back and we found it right away. The guy used a wrench and a hammer to beat the cap tight, the truck was sucking air through the crack. They gave me a new one no questions asked.