Mk2 "O/D off" flashing at random times

Hello, I'm already sorry for my english, but I hope you'll understand.

So, about a year ago I bought used ford mondeo mk2 1998 2.5 v6 and automatic gearbox, the first problem I experienced was the transmission. After driven about 100km the flashing O/D off light came up and leaving car overnight it just disappeared and I had no money at that moment to take it to repair and the gearbox died leaving me only reverse gear. So I replaced gearbox myself and been driving about 2 months without any problems. This month again the O/D off light started flashing. I read forums but didn't find anything useful, maybe I can't just search... But I assume it's an electrical problem, because this time it appears after driving about 400km or so and not always. It's totally random, and when the flashing light comes up the gearbox does these weird pushes, like hits, when I put in Drive or Reverse and in between 1st and 2nd gear on low rpm. All the other gears are smooth.

So, one day when the O/D off started blinking again I drove about 40km while it was flashing to home and took out the battery instantly and disconnected the cable for the gearbox. In that connector there was moisture. I blew it out with compressed air and connected it back, started car and everything's fine now. I've driven about 270km and all is good for now. 

My question is - could that moisture cause that problem or it could be some other electrical problem? Or did I get another faulty gearbox that's gonna die? Because gearbox oil level is good, I changed it  2 months ago when I replaced gearbox.