Using that debugger you have a long cable length that attaches to your /RST pin. In the datasheet they specifically claim "...an external pull-up and/or decoupling of the RST pin may be necessary to avoid erroneous noise-induced resets." in section 11.3. This is of course assuming the rest of the circuitry is valid. I just wonder how SILabs can make a product with an in circuit debugger that would create this problem where you have to have special circumstances. Doesn't make sense.