ValueError from read_csv when in loop but not when called independently
#### Code Sample, a copy-pastable example if possible

Given a directory with a large collection (> 1900) of files such as the attached sample ([sample_file.zip](https://github.com/pandas-dev/pandas/files/714335/sample_file.zip)), and the following pseudo-code "junk.py" (needs work to make it copy-pastable by defining dtypes and colnames, as well as generating thousands of similar files

```python
import os.path as osp
import pandas as pd

for f in ec_files:
    print(osp.basename(f))
    fdf = pd.read_csv(f, dtype=dtypes, header=None,
                      parse_dates=[0, 1], index_col=1, names=colnames,
                      na_values=["NAN"], true_values=["t"],
                      false_values=["f"], low_memory=False)
```
#### Problem description

Running this script stops at different files each time it is run with the following traceback:

```python
Traceback (most recent call last):
  File "junk.py", line 9, in <module>
    false_values=["f"], low_memory=False)
  File "/usr/lib/python2.7/dist-packages/pandas/io/parsers.py", line 645, in parser_f
    return _read(filepath_or_buffer, kwds)
  File "/usr/lib/python2.7/dist-packages/pandas/io/parsers.py", line 400, in _read
    data = parser.read()
  File "/usr/lib/python2.7/dist-packages/pandas/io/parsers.py", line 938, in read
    ret = self._engine.read(nrows)
  File "/usr/lib/python2.7/dist-packages/pandas/io/parsers.py", line 1505, in read
    data = self._reader.read(nrows)
  File "pandas/parser.pyx", line 849, in pandas.parser.TextReader.read (pandas/parser.c:9907)
  File "pandas/parser.pyx", line 945, in pandas.parser.TextReader._read_rows (pandas/parser.c:11161)
  File "pandas/parser.pyx", line 1047, in pandas.parser.TextReader._convert_column_data (pandas/parser.c:12536)
  File "pandas/parser.pyx", line 1126, in pandas.parser.TextReader._convert_tokens (pandas/parser.c:13783)
ValueError: invalid literal for float(): 2016-07-13 18:54:03.9
```

The file where the error is thrown, as well as the particular ValueError vary with each run, i.e. it is never consistent.  **Furthermore, every file where the error occurs reads without any problem with the very same read_csv command called on its own (not in a loop)**.  Therefore, the read_csv command is correct for these files.

If the number of files to loop through is relatively small (<100) the process **sometimes** finishes successfully, but not always.

I initially reported this in [Stackoverflow](http://stackoverflow.com/questions/41664844/dtype-seemingly-ignored-in-read-csv-during-long-loop)

#### Expected Output

read_csv output and behaviour should be the same whether it is run in a loop or independently.

#### Output of ``pd.show_versions()``

<details>
# Paste the output here pd.show_versions() here

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.13.final.0
python-bits: 64
OS: Linux
OS-release: 4.8.0-2-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_CA.UTF-8
LOCALE: None.None

pandas: 0.19.0+git14-ga40e185
nose: 1.3.7
pip: 9.0.1
setuptools: 32.0.0
Cython: None
numpy: 1.12.0rc2
scipy: 0.18.1
statsmodels: 0.8.0rc1
xarray: None
IPython: 5.1.0
sphinx: 1.4.9
patsy: 0.4.1+dev
dateutil: 2.5.3
pytz: 2016.7
blosc: None
bottleneck: None
tables: 3.3.0
numexpr: 2.6.1
matplotlib: 2.0.0rc2
openpyxl: 2.3.0
xlrd: 1.0.0
xlwt: 0.7.5
xlsxwriter: None
lxml: 3.7.1
bs4: 4.5.3
html5lib: 0.999999999
httplib2: 0.9.2
apiclient: 1.5.5
sqlalchemy: None
pymysql: None
psycopg2: 2.6.2 (dt dec mx pq3 ext lo64)
jinja2: 2.8
boto: None
pandas_datareader: None
```
</details>
