SSRS Independent datasets conflicting with each other - reporting-services

Question

I have a report that is acting very strangely. I thought originally the report might be corrupt somehow but was able to recreate the problem starting from scratch.
Basically, I have two datasets which both call different procs that have no connection with each other. When run through TSQL they always produce the correct results.
If I add a table to the report and show one field in a table belonging to dataset1 without adding dataset2 to the report, it always returns the correct results. However, if I then add the second dataset to the report, it will sometimes return the correct result and then on other runs return a purely random value, which is not returned by either proc.
None of the fields in either dataset have the same name. The problem is always related to the Money datatype for some reason.
Anybody any ideas what could possible be going wrong?
For Example, I've checked this with sql profiler so the correct parameter is getting passed to the proc in tsql.
I'll call proc _ proc_rpt_Test1
which will return a field called DiscountedTotalCustomerPrice. This is defined as money type in the proc. It will return a value of say 16,000 through tsql for Audit1
This field does not exist in the other dataset. I've set the table equal to dataset1 so it should not be referencing the second dataset in anyway.
So the first time you run, you will get 16,000. However the second time you run it might return 22,528, the third time 55,260. These numbers are from the previous three runs. The values being returned are not a multiple or anything, and I'm just displaying the number.
Been working with SSRS for years, and never had such an issue.
Datset 1 fields are
TotalLineItemsCount,TotalKwhSavings,NETTotalKwhSavings,TotalKwSavings,TotalKwhSavingsDollar,TotalPrice,TotalCustomerPrice,TotalUtilityPrice,CustomerDeposit,DiscountedTotalCustomerPrice,DiscountedCustomerBalance,CustomerBalance,TwelveMonthPayment,TwentyFourMonthPayment,SimplePayback,TotalKwBefore,TotalKwAfter,TotalKwhBefore,TotalKwhAfter,DatePayback,MonthlyCostOfDelay,PaybackInMonths,TotalCost,TotalMargin,QtyBeforeFixtures,QtyAfterFixtures,CurrentAnnualCost,SixMonthPayment
The second dataset was
AuditID,Incentive,DefaultDeposit,DefaultDiscount,CustomerLumpSum,MaxMonthlyPayment,CashFlowFactor,ProposedDate,MonthlyInterestRate,RawNumberofPayments,FinalNumberOfPayment,CustomerMonthlyPayment,TotalCustomerPayment,PaymentIncrements,MaxNumberOfPayments,MinNumberOfPayments,CustomerDeposit,CustomerDiscount,ActualLaborType
XML if the rectangle..as asked for
<Rectangle Name="Rectangle2">
<ReportItems>
<Textbox Name="DiscountedTotalCustomerPrice">
<CanGrow>true</CanGrow>
<KeepTogether>true</KeepTogether>
<Paragraphs>
<Paragraph>
<TextRuns>
<TextRun>
<Value>=Sum(Fields!DiscountedTotalCustomerPrice.Value, "AuditSummary")</Value>
<Style />
</TextRun>
</TextRuns>
<Style />
</Paragraph>
</Paragraphs>
<rd:DefaultName>DiscountedTotalCustomerPrice</rd:DefaultName>
<Top>0.33958in</Top>
<Left>1.17014in</Left>
<Height>0.25in</Height>
<Width>2.4375in</Width>
<Style>
<Border>
<Style>None</Style>
</Border>
<PaddingLeft>2pt</PaddingLeft>
<PaddingRight>2pt</PaddingRight>
<PaddingTop>2pt</PaddingTop>
<PaddingBottom>2pt</PaddingBottom>
</Style>
</Textbox>
</ReportItems>
<KeepTogether>true</KeepTogether>
<Top>0.4925in</Top>
<Left>0.04736in</Left>
<Height>0.8325in</Height>
<Width>8.04682in</Width>
<ZIndex>7</ZIndex>
<Style>
<Border>
<Style>None</Style>
</Border>
</Style>
</Rectangle>
From the xml for dataset
<Field Name="DiscountedTotalCustomerPrice">
<DataField>DiscountedTotalCustomerPrice</DataField>
<rd:TypeName>System.Decimal</rd:TypeName>
</Field>