MacBook Pro battery percentage jumping down and up

Recently, my 2011 MacBook Pro 13' produces strange instantaneous jumps in battery percentage. Sometimes, when it is discharging, it drops by ~40% (and if it was near or at less then 40%, it shuts down unexpectedly), sometimes (both when discharging or charging) it suddenly increases by 40%.

The output of coconutBattery shows that the maximum charge is at the half of the design capacity when the 40% is "missing":

Then the maximum charge seems to go back to normal:

System Report says:

Battery Information:

  Model Information:
  Serial Number:    W00030C0L77ZC
  Manufacturer: SMP
  Device Name:  bq20z451
  Pack Lot Code:    0
  PCB Lot Code: 0
  Firmware Version: 3
  Hardware Revision:    2
  Cell Revision:    100
  Charge Information:
  Charge Remaining (mAh):   2979
  Fully Charged:    Yes
  Charging: No
  Full Charge Capacity (mAh):   2979
  Health Information:
  Cycle Count:  86
  Condition:    Normal
  Battery Installed:    Yes
  Amperage (mA):    0
  Voltage (mV): 12471

System Power Settings:

  AC Power:
  System Sleep Timer (Minutes): 10
  Disk Sleep Timer (Minutes):   10
  Display Sleep Timer (Minutes):    180
  Wake on AC Change:    No
  Wake on Clamshell Open:   Yes
  Wake on LAN:  Yes
  Current Power Source: Yes
  Display Sleep Uses Dim:   Yes
  PrioritizeNetworkReachabilityOverSleep:   0
  Standby Delay:    4200
  Standby Enabled:  0
  Battery Power:
  System Sleep Timer (Minutes): 10
  Disk Sleep Timer (Minutes):   10
  Display Sleep Timer (Minutes):    15
  Wake on AC Change:    No
  Wake on Clamshell Open:   Yes
  Display Sleep Uses Dim:   Yes
  Reduce Brightness:    Yes
  Standby Delay:    4200
  Standby Enabled:  0

Hardware Configuration:

  UPS Installed:    No

AC Charger Information:

  Connected:    Yes
  ID:   0x0100
  Wattage (W):  60
  Revision: 0x0000
  Family:   0x00ba
  Serial Number:    0x002b46f4
  Charging: No

Should I suspect hardware failure, or is there something I might try to fix this erratic behavior?

I do not know if your CoconutBattery is doing that but the numbers do not make sense at all.

Can you look under About this Mac -> More Info -> System report -> Power and get the battery numbers from there.

What you showing here is a old battery, that has already seen over half of its life but it shows only 86 load cycles--- which is not possible/correct.

In your case resetting the SMC might solve the problem.

Here is how to:

Intel-based Macs: Resetting the System Management Controller (SMC)