I've built a windows service to sinchronize to Firebird databases.

I've built a windows service to sinchronize to Firebird databases.
A sample method to sync 2 tables is as follows:

function TCSVSync.SyncStoc(const aID: Integer; var mID: Integer; const aAction: String): Boolean;
begin
Result := False;
if aID <= -1 then Exit;
mID := -1;

if qSlaveStocs.Active then qSlaveStocs.Close;
try
qSlaveStocs.Params.ParamByName('ID').AsInteger := aID;
dbSlave.StartTransaction;
qSlaveStocs.Open();

if qMasterStocs.Active then qMasterStocs.Close;
qMasterStocs.CachedUpdates := True;
qMasterStocs.SQL.Text := ' update or insert into STOC (IDOF,IDVAL,VAL) values (:IDOF,:IDVAL,:VAL) matching (IDOF, IDVAL) returning ID ';
if (qSlaveStocs.RecordCount>0)and(not qSlaveStocs.Eof) then begin
try
dbm.StartTransaction;
try
qMasterStocs.Prepare;
qMasterStocs.Params.ParamByName('IDOF').Value := qSlaveStocs.FieldByName('IDOF').Value;
qMasterStocs.Params.ParamByName('IDVAL').Value := qSlaveStocs.FieldByName('IDVAL').Value;
qMasterStocs.Params.ParamByName('VAL').Value := qSlaveStocs.FieldByName('VAL').Value;
qMasterStocs.Open;
dbm.Commit;

if (not qMasterStocs.Eof) then
mID := qMasterStocs.FieldByName('ID').AsInteger;
except
on E: EFDDBEngineException do begin
dbM.Rollback;
end;
on E: Exception do
end
finally
qMasterStocs.Close
end;
end;
Result := True
finally
if qSlaveStocs.Active then qSlaveStocs.Close;
dbSlave.Commit
end
end;

Some times I get a error that can not detect why if it should not be: Service failed on execute: [FireDAC][Phys][FB]invalid transaction handle (expecting explicit transaction start).
Other times I get the error: [FireDAC][DApt]-400. Update command updated [0] instead of [1] record. Possible reasons: update table does not have PK or row identifier, record has been changed/deleted by another user.
Event if all tables have PK's and if try to execute the same code again it works.
What do I do wrong?

The second question is how to keep service running if any errors occurs?
Here is the main execution method:

procedure TCSVSync.ServiceExecute(Sender: TService);
var
Count,SecBetweenRuns: Integer;
begin
SecBetweenRuns := FSyncPeriod * 60;
Count := SecBetweenRuns; //:= 0;
while not Terminated do
begin
Inc(Count);
if Count >= SecBetweenRuns then
begin
Count := 0;
try
if (not DataConnected) then begin
DataConnect;
if DataConnected then begin
SyncStoc;
end
end;
finally
DataDisconnect
end;
end;
ServiceThread.ProcessRequests(false)
end
end;