iphone power problem

CMpunked

Few months back my iphone 3gs started switching off and starting back up again. Like an auto soft reset. I paid no attention to it, its happened before with phones ive had previous.
Few months later (just after ios5 came out) it starts doing it around 30-40% but this time wont switch on nearly at all unless i plug it into a charger. A bit annoying so i figured out that i had to do a full reset of the phone which i did but had the same problem happening after a few days.

Now its deciding to turn off whenever its in some sort of moderate use. Reloading a webpage, sending/recieving a text etc. I have a feeling its kind of linked to the antenna or something else as whenever it switches it back on (when its above 60ish%) it sometimes crashes again when then network status changes from searching to a single blip.

Has anyone come across this issue before or can shed some light on what i could try replacing to see if it sorts the problem?
Ive read a few fixit guides and would take a stab at replacing the battery, if that is the cause.

thanks guys.