Intel is running into problems protecting its chips from the major Meltdown and Spectre vulnerabilities that became public last week. The company has been warning customers of three specific flaws in a recent firmware update and recommending that customers hold off installing the patch, according to emails first reported by The Wall Street Journal. According to a follow-up announcement by Intel, the issue may cause reboot issues in systems running older Haswell chips.

Intel has been aware of the Spectre issues since June, but rewriting processor firmware to address the vulnerability proved to be a significant challenge. The company has committed to protecting 90 percent of its CPUs produced in the last five years, with patches to be deployed by January 15th, but technical issues have marred those patches across the board. Earlier this week, Microsoft had to halt the deployment of AMD’s Spectre patches after they rendered some computers unbootable.

Patching the CPU firmware is widely seen as the most technically difficult element of Spectre recovery, far more challenging than the operating system or browser patches that were deployed last week. It’s also the patch most likely to slow computers down, although it’s still unclear how significant the performance hit will be. Intel’s recent benchmarks show less than 5 percent slowdowns on recent processors, but those tests did not extend to the Haswell processors affected by today’s issues.