Installing MSI application via PowerShell - powershell

Question

I am trying to build a powershell script to install, an application on 15 workstations. The application requires the following:
Application process to be stopped
Previous version to be uninstalled
Install new client.
I am running into a few snags though. The uninstaller is telling me that the application isn't installed, and or the application is not a valid 64-bit application. I can run the uninstaller fine if I use the uninstall string from command line. My uninstall log file, does not tell me much, it just tells me that Powershell.
If I run through the installer manually, I reach a point after accepting the EULA, and begin the actual installation by selecting "install" that button comes up with the "Admin" shield. Automating it I get an MSI error 1603 which I cannot get past...
This is a medical application to be installed in a very restricted, environment. Thank you for your help!
Here is my code:
$SourcePath = "LOCATION OF Source"
$Destination = "C:\Temp"
$App = Get-WmiObject win32_product | where{$_.name -like "mammoscreen*"}
$Install = 'C:\Temp\tpx-client-v2.2.3.msi'
$ErrorActionPreference="SilentlyContinue"
Stop-Transcript | out-null
$ErrorActionPreference = "Continue"
Start-Transcript -path "C:\Temp\TheraInstall.log" -append
#Do some stuff and begin logging
Write-Output "Start Logging"
Get-Date
Write-Host "Copying files..." -ForegroundColor Green
Copy-Item $SourcePath -Destination $Destination
Write-Host "Stopping process..." -ForegroundColor Cyan
Stop-Process -ProcessName tpx-client-agent
#Uninstall old versions of software
Write-Host "Uninstalling previous versions..." -ForegroundColor yellow
Start-Process 'C:\Windows\System32\msiexec.exe' -ArgumentList "/X $($App.IdentifyingNumber)/qn /norestart /L*v C:\Temp\UninstallTest.log" -Wait
#Installs the new Client
Write-Host "Installing new agent v2.2.3..." -ForegroundColor Green
Start-Process 'C:\Windows\System32\msiexec.exe' -Wait -ArgumentList '/I $Install /qn /norestart /L*v C:\Temp\InstallTest.log'
#Stop logging
Stop-Transcript
#Start-Process -Filepath "tpx-client-agent"