Inconsistent custom field behaviour under some circumstances

I have 2 sites reporting erratic custom field behaviour & have identified one possible cause. The reported behaviours are
1) custom fields sometimes not showing in Search Builder
2) custom tokens sometimes not rendering

In both cases these problems come & go and are hard to replicate. I did however, find that forcing the contact::exportable fields to generate the cache for that function fixed the problem (until next time)

ie. commenting out
$fields = CRM_Core_BAO_Cache::getItem('contact fields', $cacheKeyString);

So that seems a pretty good clue that the problem is in the caching of exportable contact fields. When I look at the cache there are 3 entries (variations of the cache key - one, inexplicably, does not have any custom fields - even though the variation is on the $export var - which is a formatting variable).

I have been through the code pretty carefully & I think the problem is because both these sites have less permissioned users access the site from time to time. If that cache key was generated while a less permissioned user were logged in the fields are generated with $checkPermissions enabled But that is not reflected in the cachestring.

Note that CRM_Core_BAO_CustomField::getFieldsForImport does reflect the permissions in it's cache string.

I'm not sure that things are 'being done in the right place' - ie the permissioning seems to be 'too deep'. However, given that that altering that as a bug fix seems too much I'm thinking it is simply a case of making the cachestring more complex - the easiest thing seems to be to append the contact_id of the person doing the search to the string & apart from adding quite a few entries to the cache I'm not sure there is a downside.

The alternative would be to add this to the exportable fields function (as per the function it calls). Note sure if that would be better or worse from a performance point of view - probably much of a muchness.

$cgTable = CRM_Core_DAO_CustomGroup::getTableName();

// also get the permission stuff here
if ($checkPermission) {
$permissionClause = CRM_Core_Permission::customGroupClause(CRM_Core_Permission::VIEW,
"{$cgTable}."
);
}
else

{ $permissionClause = '(1)'; }

// lets md5 permission clause and take first 8 characters
$cacheKey .= substr(md5($permissionClause), 0, 8);