Query that normally takes 100ms, occasionally takes 30+s. Need help analyzing query plan

Question

I am using Google Cloud SQL. I am using a g1-small. I have a table that looks like this:

+-----------+-------------------+-------------+
| Column    | Type              | Modifiers   |
|-----------+-------------------+-------------|
| id        | character varying |  not null   |
| reportId  | uuid              |  not null   |
| value     | jsonb             |             |
| isTouched | boolean           |  not null   |
+-----------+-------------------+-------------+
Indexes:
    "report_variable_pkey" PRIMARY KEY, btree (id, "reportId")
    "report_variable_id_idx" btree (id)
    "report_variable_isTouched_idx" btree ("isTouched")
    "report_variable_reportId_idx" btree ("reportId")
    "report_variable_value_idx" gin ((value::text) gin_trgm_ops)
Foreign-key constraints:

I have a query that looks like this:

INSERT INTO
  report_variable ("id",
    "reportId",
    "value",
    "isTouched")
SELECT
  *
FROM
  UNNEST($1::"varchar"[], $2::"uuid"[], $3::"jsonb"[], $4::"bool"[])
ON
  CONFLICT ("id",
    "reportId") DO
UPDATE
SET
  "value" = EXCLUDED."value",
  "isTouched" = EXCLUDED."isTouched" RETURNING *

My table has ~1.6m rows in it. This query runs thousands of times per day. Occasionally (as in, every 100 executions?), it will take a VERY long time. Here is a graph showing latency:

Here is the data that comes from the Google Cloud SQL Query Insights page, for an average query:

Here is the data for one of the strange queries:

Some things I noticed:

• Why is I/O Read Time so much higher?
• Why is Shared Dirtied Blocks, Shared Hit Blocks, Shared Read Blocks and Shared Written Blocks? Why are they so much higher?

How can I find out more information? Let me know if you need any more info. Thank you in advance.