Time loses (Normal mode) in every 10 min

i am facing a problem in win98 system. The m/c is IBM pc 300PL(PII). Problem is the time loses in every 5-10 min under Normal mode of win98 .it keeps on changing back and forward by 1 or 2 hour after setting current time. It works fine in Safe mode & Dos mode.For resolving it, i have replaced the CMOS battery as well as motherboard successively but the problem is still existing. Also, i have updated bios and disabled APM in bios but no luck. so i think there is some s/w problem which i am unable to isolate. so please can u help me out in solving this problem by sending appropriate solutions.i will highly appreciate for that.