There is a possibility that the largest electrolytic capacitor in the power supply may be failing or has failed. Examine the capacitor for a bloated or split top. Replacement of that $2 capacitor takes extensive disassembly, about one hour labor. This is unlikely to be the problem if the DVD Drive still "spins-up."
When there are read, write or finalizing problems, perhaps even the U61 or U88 errors, be sure to listen to your DVD Drive. What do you hear? If there is a "grinding" noise the problem might be as simple as a dirty rubber hub in the DVD Drive.
An U99 error generally follows the U88 and U61 errors, often where a DVD Drive makes "clunking," "chugging" and "errrp" noises. The problem might be related to a DVD Drive with a dirty lens but most often there is a failed laser assembly.
Other threads describe the routine lens and rubber hub/spindle cleaning, a ten minute job with a 2006 DMR-ES35V. If the problems persist the laser assembly has most likely failed.
The U99 is specifically a "hang up" that is displayed when there is a communication error that has occured between the main microprocessor and the timer microprocessor. That indicates a problemtic Digital PCB, the "brains" of a Panasonic. The Digital PCB in a DMR-ES35V is the board mounted on the sub-platform underneath the DVD Drive. Replacement is indicated. The Panasonic approved service procedure for 2006 models (like the DMR-ES35V) is replacement of the DVD Drive and the Digital PCB together as a "module."
The $130 flat-rate repair (including parts and labor) through Panasonic's corporate Service Center in Elgin Illinois should be considered before approaching an "authorized" local Panasonic shop where this module, described as the "RFKNES35VP RAM DIGITAL PCB MODULE," is currently priced at $258.67 for the parts alone. Add another $100 or more for shipping the parts and diagnostic, replacement and testing labor.
While the DMR-ES35V is an outstanding product (I own six DMR-ES35V models, three of which have each accumulated more than 3,000 recording hours) it may not be cost effective to repair this model, especially when the outstanding Magnavox 2160 HDD/DVD recorder may be purchased for $159.00 (like-new refurbished) at jr.com or new for $198.00 at walmart.com.