HHT -code 35 is O2 sens at rich limit , lean condition.
[assuming he got the HHT from the HFM-SFI module]

Possibles are fuel pressure regulator or vac leak,
Because it does it at certain times, I would check for vac leaks AFTER the switch-over valve at both air pump and egr valves.
Also purge valve..and all hoses...
On the reg., look for fuel drip when removal of vac line to reg.....