1998 Mercury Sable Car has intermittent trouble uphill.

Hello. Well, the main problem my car is having is sometimes when I go up a hill or slow incline that's maybe a half mile or longer, highway or around town, the car starts to shudder, it stops accelerating, the RPMs seem to slowly drop and the 'service engine soon' light starts flashing. About a minute after I crest the hill or incline the 'service' light stops. This has occurred on about 8 or 9 occasions, but doesn't do it all the time. I have not had this problem on flat surfaces or small, short hills. I've also noticed that the car idles rough when stopped at a light and it doesn't seem to have the acceleration it used to. I brought my car to a garage after it happened the first 3 times and they thought it was the fuel filter, which they found to be clogged and thus replaced. A few days later the problem happened again. I returned to the garage and they hooked it up to their computer and no error codes came up. They were stumped as to what the problem could be, except maybe that the spark plugs were misfiring, but I had an ignition tune-up 5 months prior at another mechanic. So, I brought my car back to said mechanic to have him check the plugs and wiring and they said it was all fine. I told them about the problem I'm having, they hooked it up to their computer (no error codes again) and took it for a ride and they also told me they could not pinpoint the issue either. The best they could tell me was that I could spend a lot of money to possibly find the problem. So here I am, still looking for answers. Any suggestions?