Epmp 2000 ethernet issue

We have a Epmp 2000 lite that about once an hour the ping times start to climb from 1ms from switch to radio to 200 to 300 ms.  Client can no longer pass traffic and if left long enough can no longer log into AP.

We have replaced cable, ends, swiitch, tried different switch port. 

This AP has one customer on it and they are isolated in the switch from all other traffic.  This is happening at a regular almost 1 hour interval.  There is no data in the crashlog and we are on 3.3 for firmware.

Any Ideas?

Whats your grounding situation look like at this site ?

Small ROHN tower attached to a steel building everything is grounded.  None of the other equipment we have there has any problems.  It is not our tower so I cannot do much else to the grounding.

Any inline surge protectors ? Had some axis cameras behaving similarly a while back and it ended up being bad arrestors.

Currently no surge suppresors.  Shielded cat 5 about 20 feet from poe switch. Have tried different ports and replaced ends.  Cable tests good.  Other equipment on same switch and tower no issues at all.  The ping times start slowly climbing about 15 minutes after reboot.  After an hour or so traffic almost does not pass through to client.  After a few hours can no longer access the gui and have to do power cycle on unit.  No ethernet errors in switch or radio

Went to the site and replaced with an Epmp 1000 and problem has gone away.  I am going to test this unit at the shop and see if I can reproduce the issue