If what your are describing is wow and flutter or slippage, perhaps it is a dirty tape path, heads, guides, capstan shaft, pinch roller? Tape path is the front line to good sound with tapes of any type and if the path is compromised or dirty, everything downstream is affected. Maybe post a photo after cleanup. Ronsonol (ZIppo) lighter fluid is perfect for the rubber roller. Denatured or 99% isopropyl for the heads, guides and capstan shaft.
Once we eliminate the tape path, a systematic approach is used to diagnose. When you replaced the belts did you do all of them, is there an idler tyre? Idler Tyres are a leading cause of weak pull, fast wind, and other transport issues, same as belts. I have a Ct-F500 here for service and I need to get into it sooner than later. Perhaps if the issue is deeper I can help you get yours sorted while going thru the one I have here.
Let me know how the tape path is after cleanup. Excess oil, lube, DeOxit is the bane of transports, get it on anything and you have issues. . Everything needs to be clean and adusted properly with decent belts, idler tyre and pinch rollers. Rollers are not a problem, I have them here and if I don't have the right ones, I get Terrys Rubber Rollers to re-rubber the old one.
