I'll have to weigh in on this one, I just replaced the reed switch on my 7yr old Vantage Pro. I found mine had been acting up kind of like you are experiencing, then all of a sudden I started getting 25" downpours on sunny days! looking it over there was small amount of corosion but nothing serious.. I really think the switch plain went out, but maybe the connection was going bad too (as may be on yours).
So here is how the replacement went for me:
You can get the replacement direct from Davis fairly reasonably priced ($2):
http://davisnet.com/weather/products/weather_product.asp?pnum=07120.031.
Observations: the reed swx is in a "cradle" under the tipping bucket and held in place with a small plastic cover. I pried the cover off it up on the roof because I didn't feel like taking the whole thing down. Release the cable from the ISS, loosen up the radiation shield and snake the attached cable on up through the access hole and it's free. Then I brought it down to the workbench and got the old man glasses out. The reed switch has a little bump on one end (smaller) take note of how that is oriented and the correct end of the wire. I also noted the position of the reed itself inside the casing, flat side up and could distinguish what was top vs bottom on the original. Then it was just a case of heating up the old solder to release the old switch and carefully re-soldering the cables to the new one ensuring the exact same positioning as the original.
A couple of notes, the replacement has much longer ends than the installed one, I of course gave a little more room on each end to make it easier.. wrong. back up on the roof and back down, re-soldered one more time with appropriately short ends so it would fit back in the cradle correctly! also the cable is just kind of "spot" soldered on the leads, pretty difficult to do. I gave the cable ends a twist around the sensor leads to hold in place and hopefully give a better bond soldered. back to the roof, put it back in the cradle, snap the cover back in place, bring the cable back through the shield and back into the ISS. Easy as Pie!
Works perfectly now. Wife just reported .46 on the CoCoRaHS, and I see .43 on the station.. hmm I may have to double-check her manual reading!