Initially I had the simmilar problems with my Sandisk 2GB card.
Then I discovered that PocketMax Phonealarm 1.57.00 caused problems. After I went back to Phonealarm 1.56.10 everything worked fine again. The most recent version of Phonealarm is 1.58.00 but I did not try that version yet as 1.56.10 works fine for me.
To see if your Phonealarm is causing problems just try to uninstall, softreset and use your device for a while without Phonealarm.
HTC LEO : HD2
ROM: 1.66 WWE - RADIO: 2.06
3G T-Mobile flatrate internet
Past: Many older Nokia's + HP-Compaq PPC's -> Qtek8060 -> Typhoon -> Magician -> Wizard -> Trinity -> Polaris + Shift -> Raphael -> Blackstone + EeePC 1000HE