thanks for the replies. The factory firmware is the latest & greatest. the contact tips will certainly get a closer look too. Batteries have been an issue (true almost everywhere) They have an in house maint man who waters & checks the batteries. they use the opportunity chargers.
I locked one of the problem trucks to a charger & made sure it got a full equalization charge & it is much better now. I check the batteries myself when we work on them & the specific gravity, volts per cell & amps are typically in the correct range. That being said they are 4 year old batteries & will likely start failing soon.
It just seems odd that with the same trucks & around the same amount of hours (approx 5,000) on all of them that the trouble would limited to just these few.
I will post again after a follow up in 2 weeks
Thanks.