Just a suggestion, try installing the system with the CD/DVD install71.iso
During installation you will be asked if you want X (gui) login, or if not, at the prompt
you can manually, as non-root startx
If successful post your dmesg file here for further scrunity by those who can look at your system particulars.
https://www.openbsd.org/faq/faq4.html#SendDmesg