Updating macro to Excel 2010

I have a macro which was developed in Excel 2007 (version 12.0?). When I run it in Excel 2010 (version 14.0), I get some inconsistent errors as follows:

I have this code operating on one worksheet:

    Range("A1").Select
    ActiveSheet.Paste

which is acceptable. A little later, I have, operating on a different worksheet:

    Range("A6").Select 
    ActiveSheet.Paste

which gives a run-time error '1004': Paste method of Worksheet class failed.

Commenting that statement out and running again. I have this code:

    Cells.Find(What:="3", After:=ActiveCell, LookIn:=xlFormulas, LookAt:= _
        xlPart, SearchOrder:=xlByColumns, SearchDirection:=xlNext, MatchCase:=False) _
        .Activate

which runs OK. A few lines further on:

    Cells.Find(What:="6", After:=ActiveCell, LookIn:=xlFormulas, LookAt:= _
        xlPart, SearchOrder:=xlByColumns, SearchDirection:=xlNext, MatchCase:=False) _
        .Activate

fails with a run-time error '91': Object variable or width block variable not set.

What's happening and how do I fix it? These appear to be the only sources of run-time error.

I'm running the macro in Windows 7 on an ASUS Eee Slate. I can't test it anywhere else.