How do I test the sticks to determine which I need to be replaced

Ok, I have answered this question so many times, that I feel it needs to be in print.

The lack of detail is intentional, as this should not be attempted by anyone that cannot understand what I have written.

The IMA big switch needs to be off for all but the last full balancing grid charge.
This pack can kill you, so don't attempt this if you are not qualified working on HV equipment.

After you pull the pack,you will need to open both ends so you can remove the sticks.
Carefully remove all the screws on each side.
The relay side has rubber plugs that need to be removed before you can get to the connection screws.

On the orange end connector boards, the the little screws are for the PTC strips.
Be gentle, and don't bend the small PTC tabs any more than required to get the plates off.
Once the screws and end plates are off, you can pull out the sticks.
On a couple of the sticks you will see the temperature sensor wires.
Beware, if you try to pull out these sticks, you will break the thermistors, so slide the sticks toward the wires until you can cut them loose from the tape that is holding them, then pull out the stick.
Record where in the pack they were the temp sensors are located, as you will put them back when you re assemble the pack.

You need to test each stick to see how many AH it can produce, and how well it holds a charge over time.

RC hobby shops have special charger/conditioners, that do a nice job of automatically cycling sticks of NIMH cells
Many to choose from, and I can't make a suggestion so you need to do your homework.

After you cycle each stick 3 times and record the Ah on each discharge, give the stick a number, and record the AH, time and date of the last discharge, then one more charge. Put the cell on a room temp shelf, and one week later you will discharge the cell and record the AH.
During the cycling, you will likely see the AH increasing on each cycle, this is normal for NIMH cells, and is why no serious RC electric modeler will be without one of these charger/cyclers.
After 3 cycles, you will likely have reached the max AH recovery.
You should see a range of values.The smaller the range the better the pack.
The best could be 5-6AH, and the worse could be less than 2AH.
At this point, anything below 4 AH may want to be replaced, but if the whole pack is at 4AH, you can still use it, as long as all the sticks are in the same range. One stick that is at 2 AH,in a pack of 4AH cells will likely have problems, but a pack of well matched 4AH cells and an occasional grid charge , and you may be ok for years.
The match between the sticks is more important than the actual AH, to a point.Once the Ah gets below 3AH, the pack will not provide much assist before hitting empty.

My numbers are not meant to be taken literally, only as a guide.
3 cycles is all that you should need to do on each stick to get to the best capacity.

After a week of sitting in the charged condition, do a measured discharge to see how much the AH is different from the last discharge the day of the Cycling.
A good stick may be within 5-10% of the last discharge after cycling. A bad stick, with high self discharge will be much lower in capacity, The matching of sticks requires that the % of residual charge after a week delay should be similar on all sticks.The best sticks will have the smallest % difference from the last charge on the day of cycling.
Get a replacement pack and test all those sticks the same way, and after it is all done, you pick the best matched and highest AH sticks to put in your pack.
reconnect the sticks making sure to put them in with the correct polarity, and when attaching the orange end plates, make sure to carefully feed the PTC wires through the slots and reconnect securelt with the small screws. The big 10MM bolts on each stick end need to be tight to assure good connections.
When the pack is all together.
At this point, turn on the switch, and give the pack good balancing charge to bring all of the subpacks to full.
Turn off the switch, and put the pack back inthe car, and you will be good to go.
You should see your IMA problems solved.
While you have the pack out of the car, you may want to install a charger harness, so the pack can be rebalanced by one of my chargers.
Any sticks that drop more than 30% after a week are on the edge as far as being good for a long term replacement, but a pack of sticks with similar capacity and self discharge can be kept operational with a weekly balancing charge of the whole pack.
Good luck