Speedo problems from weak battery?

90' 348ts. Before I had a problem that if I was driving down the road my speedo would max out and start running up the miles on the odometer. If you gave the dash a good smack it would go back to normal and function properly. I took the console out and repressed all the ribbons and cleaned the connections. I also cleaned the sending unit and since I've done that I hadn't had any problems until now. My battery tender came unplugged and my battery died while I was away for 4 weeks. So I charged the battery for 15-20 mins and then jumped it. On my way down my driveway to take it out for a recharge spin the speedo has the same manlfunction that I haven't seen in awhile. Any thoughts?

 

One thing I forgot to mention is that after I put a full charge on the battery it worked fine (so far). I just hate the thought that it could screw up at any moment and rack up miles.