Performance degradation after upgrading from 4.2.4 to 5.2.5

Hello,

After upgrading from 4.2.4 to 5.2.5 we started to experience significant performance degradation in our YouTrack standalone installation. Here are our "Issues" page generation timings:

Created in 43663 ms
Created in 28193 ms
Created in 31059 ms

Moreover, after some time, YouTrack instance become completely unavailable. It even does not respond to service stop command. After restart (with process termination) it starts very slowly, in order of several minutes.

I tried to increase MaxPermSize value to 250m and Maximum Memory Pool to 1024Mb (from 512Mb). This step increased lifetime (and sometimes page generation times) of the YouTrack instance, but didn't solve the problem.

Here is our statistics page:

Available processors    4
Available memory    989.8 MB
Allocated memory    989.8 MB
Used memory    965.9 MB
Uptime    45 minutes, 8 seconds and 530 milliseconds (started at Tuesday, October 7, 2014 12:45:10 PM +04:00)
Online users    Users: 3 (max is 3, moments ago (13:29))
Sessions: 4
Windows: 9
Database size (without blobs)    236.4 MB
Text index size    25.3 MB
Database background threads    2
Pending asynchronous jobs    0
Number of cached results in the database queries cache    1812
Database queries cache hit rate    63.40%
Blob strings cache hit rate    0.00%
Total transactions    140
Transactions per second    0.05
Requests per second    0.59
Notification Analyzer Queue Size    0
Notification Sender Queue Size    0
User Action Job Processor Queue Size    0
There is no job being executed

I've also attached "errors.log" file - it contains some OutOfMemoryExceptions.

Please help, we can't work!