Transport going down? Checklist suggestions

Hi there!

I´m using a Pioneer DVD 606 as CD transport.
A couple of weeks ago I noticed a high frequency distortion that started to appear after 4-5 minutes of playing. This initially happens with sound above 2000hz and is worse as music frequencies go higher. As playing continues the distortion increases to unbearable levels.
Setting the player to rest for a couple of hours makes it work fine initially but the cycle repeats.
Following things have been done without success: lens cleaning, cleaning and relubing laser rails. Leaving the unit unplugged for two days and trying after produces same results.
Once in a while a disc plays with no problems but changing to another disc does produce the distortion. Both disc worked fine before and work OK in friends transports.

Could be a laser alignment problem or is an electonic part of the circuit giving me the problem.
Please share your ideas about where to look in solving this.

For the time been I´m in a must keep for a couple of months situation then I´ll upgrade....
Thanks
Luis