Reusable SteppablePipeline code breaks randomly - powershell

Question

I have been writing a Powershell library which uses a lot of wrappers around one or two functions. Thanks to a previous question, I discovered that I could use the GetSteppablePipeline() method to do the wrapping. However, I would like to be able to generalize this code. Googling the command, I discovered that there are only one or two samples which have been copied to death, but no single function. So I tried it. It seemed to work at first, and the sample below is roughly what I created first. But when I started to use the code in anger, it just seemed to break all over the place.
If you run the below straight through, it works ok. But if you put in any breakpoints in the ISE, it will break with errors like "Cannot find drive. A drive with the name 'function' does not exist.". If you incorporate the code into any other script, it will also break with seemingly incomprehensible errors (for instance, Where-Object not being recognised as a Cmdlet).
Function CreateSteppablePipeline
{
Param(
[Parameter(Mandatory = $true, Position = 0)]
[string]
$InnerFunctionName,
[Parameter(Mandatory = $true, Position = 1)]
[System.Collections.Generic.Dictionary`2[System.String,System.Object]]
$OuterPSBoundParameters,
[Parameter(Mandatory = $true, Position = 2)]
[System.Management.Automation.EngineIntrinsics]
$OuterExecutionContext,
[Parameter(Mandatory = $true, Position = 3)]
[System.Management.Automation.InvocationInfo]
$OuterMyInvocation
)
$outBuffer = $null
if ($OuterPSBoundParameters.TryGetValue('OutBuffer', [ref]$outBuffer))
{
$OuterPSBoundParameters['OutBuffer'] = 1
}
[System.Management.Automation.FunctionInfo] $wrappedCmd = $OuterExecutionContext.InvokeCommand.GetCommand($InnerFunctionName, [System.Management.Automation.CommandTypes]::Function);
[ScriptBlock] $scriptCmd = { & $wrappedCmd #OuterPSBoundParameters };
[System.Management.Automation.SteppablePipeline] $steppablePipeline = $scriptCmd.GetSteppablePipeline($OuterMyInvocation.CommandOrigin);
$steppablePipeline.Begin($OuterMyInvocation.ExpectingInput);
$steppablePipeline;
}
Function Inner
{
[CmdletBinding()]
Param(
[Parameter(Mandatory=$true, Position=0)]
[string] $ExParam,
[Parameter(Mandatory=$true, Position=1, ValueFromPipeline=$true)]
[string[]] $Value
)
Begin {
[int] $i = 0;
}
Process {
$i++;
ForEach ($myValue In $Value)
{
$myValue;
}
}
End {
$i;
$ExParam;
}
}
Function Outer
{
[CmdletBinding()]
Param(
[Parameter(Mandatory=$true, Position=0)]
[string] $ExParam,
[Parameter(Mandatory=$true, Position=1, ValueFromPipeline=$true)]
[string[]] $Value
)
Begin {
[System.Management.Automation.SteppablePipeline] $steppablePipeline = CreateSteppablePipeline "Inner" $PSBoundParameters $ExecutionContext $MyInvocation;
}
Process {
$steppablePipeline.Process($_) |
ForEach-Object {
$_ + " : Pipelined";
}
}
End {
$steppablePipeline.End()
}
}
Outer -ExParam 22 "Name1","Name2","Name3","Name4","Name5","Name6","Name7"
"Name1","Name2","Name3","Name4","Name5","Name6","Name7" | Outer -ExParam 22
It has probably to do with scoping (a problem I have come across a number of times in scripts). I tried moving parts of the CreateSteppablePipeline() function between the calling and function scope, but with no luck.