Senna
I think you should fix what you have first before spending any money.
What Christop and Krasw said is correct and I agree.
It sounds like you have a incorrect polar installed in your computor or a pneumatic tubing leak.
Your Netto function should work much better than you describe.
Double and triple check you have the correct polar data entered.
Mike Borgelt on his website has a tubing leak repair and test guide.
Lee Kuhlke also wrote a excellent article on testing in Soaring a few years back.
You only need a few dollars in tools and an less than an hour to do this.
You can't just look at the tubing; you Have to check it for leaks with a gauge, likely your airspeed indicator. This plumbing leak test is easy to do, finding and fixing the leak, if you have one, could take more time, but you have to have a leak free system before anything is going to work well.
These gliding computers function off of very low pressure's and suction.
Ask someone who knows if your TE probe is adequate.
Fix what you got and save that money for tows, or chocolates for your girlfriend the next time you leave her for a soaring trip!