What Gerhard said but something extra: it could well be the field. I've had field trouble on a Makita HR2220 rotary hammer and on a 3612BR router - both of which I picked up in eBay. Luckily I have a number of both of these machines and was able to determine the problem through trial and error.
Nice too 'cause a field is a lot cheaper than an arbour...
Is it wrong to be in love with a sawbench?