Unable To Execute Multiple SSIS Packages From Windows Service

Need some help...
When we tried to run mulitple packages one after the other from a windows service, first one succeeds but later ones are throwing below error :
"The script threw an exception: The element cannot be found in a collection. This error happens when you try to retrieve an element from a collection on a container during execution of the package and the element is not there.
A deadlock was detected while trying to lock variable "System:ackageName, User::BusinessDate, User::Environment, User:ortfolioName" for read access. A lock could not be acquired after 16 attempts and timed out."

Later, we tried to create separate AppDomains for each package and execute via console application, but ended up with below error (The below expressions were defined in OnError Event) :
"The result of the expression "@[User::ReportErrorFrom]" on property "FromLine" cannot be written to the property. The expression was evaluated, but cannot be set on the property.
The result of the expression ""Error At :" + @[System:ourceName] + "
" +
"Error Description : "+ @[System::ErrorDescription] + "
" " on property "MessageSource" cannot be written to the property. The expression was evaluated, but cannot be set on the property."

At last, we tried to span a separate process (System.Diagnostics.Process) for each package. this seems working but taking very long time:
A package that normally takes 2 min, is taking 60 min.

We also tried creating an SSIS Package that executes mulitple packages. But only first package is getting executed, and second one is throwing below error (Here the variable it is trying to lock is of first package):
"Failed to lock variable "UniqueInstrumentsQuery1" for read access with error 0xC0010001 "The variable cannot be found. This occurs when an attempt is made to retrieve a variable from the Variables collection on a container during execution of the package, and the variable is not there. The variable name may have changed or the variable is not being created.".

Please help us with some work around for this. Thanking you in advance,