Pacemaster Platinum Pro won't get up to speed over 5 mph when running

Have had a Pacemaster Platinum Pro for 12 years, but only used for roughly 5 years (moved to LA area and it was in storage). Have recently put it back into use and noticed that the belt was visibly slipping and could feel it when on the treadmill. Tightened the belts (both drive and deck). It operates just fine when walking on the belt up to 5 mph speeds. However, for any speed over 5 mph and approaching a run the belt never gets up to speed. The speed indicator light keeps blinking. I can race walk and get it up to speed, or if I step off the belt, it will noticeably speed up and the speed indicator light stops blinking. However, when stepping back on the belt and/or attempting to run, the belt slows down, but the speed indicator light stays constant.

I looked at the troubleshooting on your site (very helpful - thank you), but want to be cautious and not overtighten the deck belt. I don't believe the drive belt is too loose/tight - can twist it to 90 degrees with some effort. I can also pull the deck belt up 3" in the middle.

I'm ready to try whatever you would recommend.