I have a P0300 code for a random misfire.

change rotor and distributor i have 2000 gmc sierra 4.3 I had to change distributor and rotor to stop random misfire

Fuel pressure being low is a concern and could definitely cause a misfire. Is the fuel pump volume OK it should put out a pint of fuel in 10 seconds. Mis-fires could be caused by intake manifold leaks as well. A professional grade scan tool would read live sensor data but also long and short term fuel trim figures which may locate where the problem is. Autozone have some popular workshop manuals for free on line once you register (for free) on their website http://bit.ly/autozone_repair_info. There is a company that independent auto repair shops get their information from, this same information is available inexpensively for people that work on their own cars.. The information is year make and model specific, covering repair procedures, torque specifications, fluid capacities and specifications, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online I just had a Toyota Sienna in the shop a few weeks ago, it was really hard to start ran awful when it did start, I put fresh gas in it it set various cylinder misfire codes that changed from a few injectors to different injectors. It turned out his kid had put gas in in from cans that were in there garage at home. I put another big inline filter in line (had to cut the fuel line and put it in series) I did an injector rail flush and now car runs great. There is a big difference in scan tools, professional grade scan tool with Mode 6 and freeze frame/live sensor information is a great help in looking at complex problems but as you are trying to do don't over look the obvious simple things. Alldata is great for theory of operation and test procedures.

could have bad wires us best plug wires that u can buy an ckeck for cracked plug use platum plug autolite or acdelco spark plugs