A few years. I was having this problem about. Year ago and put the car up, and just now getting it back out to figure out the problem.
They are 7 cell. So it kind of sounds like probably bad batteries.
I have a Traxxas Bigfoot. I have two NIMH batteries. I can put them on the charger and they show to be charged after just a few minutes. Voltage is 8.7 volts. When I put them in my truck it runs as if it’s under powered. Won’t spin the tires or do a wheelie. After about a minute it just stops...