Remove the platter and check the tightness of the bolts holding the motor up to the top plate, if the motor sags it can effect the position of the speed pulley
Then check the tiny grub screws on the speed pulley and the magnetic speed brake disk, if these have moved out of vertical position it effects the speed
With platter off press play and check how the idler wheel lines up with the speed pulley as Geoff says it could be just out of alignment, check it at each speed
Whilst you are there clean the inside of the rim of the platter up to a bright polished finish and degrease with isopropanol
Clean the idler wheel rubber , I use my R2R pinch roller cleaner.
Oil the main bearing and a single spot of oil in the idler wheel bearing
Check the mechanical linkage that changes the speed is lubricated with grease
Alan
Turntable - Garrard 401/Jelco 750L/Ortofon Kontrapunkt B, Pioneer PLC 590, Micro Sieki MA505 , Denon DL103R - DIY Paradise Phono stage - Reel 2 Reel Studer A810, Otari MX55,Tascam BR20, Revox A77, B77, PR99, TEAC X1000 & 3440, Digital HTPC / Young Dac - Preamp - DIY B4, 821, Power Amp's DIY Avondale NCC300 Mono Block, Speakers Wilmslow Kit Volt BM220.8 / Scanspeak D2905/9500