Exclusive lock question

I have been trying to diagnose a performance problem we have been seeing with
a postgres application. The performance of the database server is usually
quite good but every now and then it slows to a crawl. The output of vmstat
does not show excessive CPU usage or disk IO. The output of ps does show that
the number of postgres process's that appear to be stuck in some query spikes
and in some cases restarting the postgres server is the only way to clear
them. While trying to diagnose this problem I ran

select * from pg_locks

I could understand most of the output but I was wondering what a result like
the following indicates

relation | database | transaction | pid | mode | granted
----------+----------+-------------+-------+---------------+---------
| | 26052434 | 29411 | ExclusiveLock | t
| | 26051641 | 29345 | ExclusiveLock | t
| | 26052415 | 29519 | ExclusiveLock | t
| | 26052407 | 29381 | ExclusiveLock | t
| | 26052432 | 29658 | ExclusiveLock | t

When I see the slowdowns there are hundreds of these with no entry for
relation or database. Any ideas what is being locked in this case?

Emil