Truck Repair: 1996 blazer start problem, oil pressure switch, water remover

Question
I have a problem with this 1996 Blazer
4X4, 4.3l eng. 70k miles.
just about 1 week ago cranked, but did not start, battery seems to be ok), so I sprayed using starting fluid into throat-surprise, starts, no problem. Later I,ve added water remover, and fuel injector cleaner into gas tank. Now,depends on which day, I need to turn the ignition key few times (on-off) before it starts, when wont start-I spray starting fluid (when is colder outside seems hesitates more or wont start normaly, I mean without the fluid). Friend told me - needs new fuel pump, fuel filter because when starts does not accelerate in first moment. Also told me I need to replace the spark plugs.
Do you think could be that problem, or something else?
It's funny, but when I do use battery charger to start, no problem, starts easy)
The cost to replace the fuel pump is high, can I replace by myself?
Is it any relay should I consider to check first? tell me how easy to replace the sparkplugs?

Thank you for the advice.
chris  

Answer
Hi Chris,
Hmmm...you say battery "seems" ok. But then later you say it starts with no problem when you use a battery charger.
The pump itself, and the relay that operate it are both voltage dependent, and if the battery, or cables are questionable, they may operate intermittently.
I was going to suggest the relay, until you mentioned the battery charger.
Now, the fuel pump is also powered by an oil pressure switch, that supplies battery voltage to the pump as long as the oil pressure is present, weather the relay works or not. That would explain why it would start on fluid, then run and start fine.
It could also explain why it would start after several key cycles...you are coaxing the pump to work.
But I would try cleaning the battery connections, checking the battery voltage....before charging or cranking. After the truck starts and runs a minute, the battery is more charged than before.
If it is the orriginal battery, I'd probably pull it out and try a new one.
Then, before replacing the pump, at least check the pressure, and filter first.
Good luck,
Van