npm install sometimes fails in Visual Studio Online - powershell

Question

I had problems with VS Online and nuget-restore. It would report that it couldn't find packages, which Visual Studio 2015 found and installed. So I specified in nuget's targets file to use APIv3 and APIv2. Since than, all nuget packages are always found, but I'm starting to experience different, rather random, errors with npm.
I've a pre-build script (configured in my project file) which installs all necessary npm and bower packages, and afterwards runs gulp. This works fine locally, and most of the times also online. But every ~4th time on average, I get an exception similar to this:
EPERM, open 'C:\NPM\Cache\fedb6d47-PM-Cache-clone-1-0-2-package-tgz.lock'
or this:
EPERM, open 'C:\NPM\Cache\ca5822dc-sh-isarguments-3-0-4-package-tgz.lock'
I was working on the gulpfile in the beginning, so I thought some of those changes caused it, but now, I'm not even touching that project anymore, and it compiled fine until suddenly I got this error again.
In the meanwhile, I've added a tiny change to commit and push it, and it compiles fine again. Any idea what can cause this and how I can at least reduce the risk of getting that error?
Here is my pre-build powershell script:
param($build_config)
$webProjectName = "WEB_PROJECT"
$scriptpath = $MyInvocation.MyCommand.Path
$dir = Split-Path $scriptpath
cd $dir
cd ..\src\$webProjectName
echo "npm install"
npm install
echo "bower install"
bower install
echo "gulp $build_config"
gulp $build_config
Update:
I used to execute that powershell script via
PowerShell -File script.ps1
and after switching to
PowerShell -NonInteractive -NoProfile -Command script.ps1
it seemed to have fixed the error, however after ~20 successful builds, I got again a similar problem (although much less often now)

Answer 1

Further investigation shows, there might be some problem with the network connection or a proxy, which is weird, because bower install always succeeds. Anyhow, the solution which is working quite well for me is the following:
use the native npm install build step from visual studio online as a first step
add an environment variable to the build service in order to identify in your script, that it's running on the build server (I called it VSO and assigned it the value 1)
check in your script if it's running on the build server or not, in powershell you can do it like this:
If ($env:VSO -ne 1)
{
echo "npm install"
npm install
}